All that is good in theory. And is about the theory that the designers of the F4 Phantom came up with. After all since we now have missiles what do we need guns for? The problem the fighter pilots found is that missiles have a definite engagement envelope, anything outside that envelope is going to miss. Anything at the edge of the envelope is likely to miss and an aware and gutsy target can avoid a missile using last second maneuvers. Now a storm of missiles, with different velocities and initial target points and staggered launches is a different matter. However at the closing speeds you are talking about you are limiting your enevelope. remember Space is a big place, missile warheads are relatively small, especially if you can maneuver in 3d, you don't need that much of a jink to generate a miss. You will, of course, always have to time your jink or break maneuver correctly, but computers are good on both sides of the equation, all I have to do is to make sure I jerk my ship out of your blast radius/proximity detonation range at the right time and your missile explodes harmlessly or goes blazing past with no hope of turning around and coming back to me. If the missiles are coming in in a cluster which is a snapshot environment they will have to because there isn't time to stagger launches then you are avoiding a cluster of missiles the same way as you were avoiding a single missile in a fighter.
In the Honor Harrington series they have missile duels but in those books the missiles have incredible speeds and it is more like a missile strike against wet water ships. (Basically a missile strike against a relatively stationary target.) You are attempting to swamp the point defense and overload other defenses.
Traveller has the missiles at the same speed and accelleration as a ship. (Against Top of the line Naval ships anyway.)
Most Traveller combat that I have seen where the missiles were used as 6G small ships, the only place where they were actually effective was a tail chase situation where the launching ship had a velocity advantage or the missiles were launched at such short range that they hit in the launching turn. There is your snapshot but only one side will get it. because the other side gets past during their movement and you are now in a situation where your missile can't catch up. (Basically the ideal range for missiles in Mayday is 6 hexes + your initial vector or less.) Which is what you are envisioning. Then it comes down to how your movement sequence works, because even though it is a one turn strike one turn is 20 minutes and maneuvering is going to happen. And only one side will be able to get a shot in.
To put it into something, perhaps easier to visualize, it is kind of like the Quarterback sidesteping and dodging a safety blitz.
Or taking into account an actual Naval engagement. From the reports I read, not saying they are true or not, the loss of the HMS Sheffield in the Falklands went something like this. Two missiles were fired at short range. Point defense automatically tracked and identified the targets. However point defense identified the targets correctly as EXOCENT missiles and classified them as friendly (obviously incorrectly) and failed to engage. The Skipper, realizing his ship was in trouble executed a break maneuver (Though relative agility between a missile and a ship is a joke.) and blew a chaff pod. One missile was distracted by the chaff and missed the other was not distracted and struck the ship in the vicinity of CIC and primary damage control destroying damage control coordination and knocking out most of the fire fighting equipment. The remaining fuel onboard the missile burned at such a temperature that the Aluminum superstructure caught fire and burned. Translated into Traveller it is obviously a critical hit, with secondary damage effects resulting in the loss of the ship.
Like I said I am not sure how accurate the report I read was, however immediately after the Sheffield was destroyed the US stopped building Aluminum superstructure ships and the Phalanx (And the British equivalent, goalkeeper I believe) was developed and immediately deployed and placed on virtually every ship in the inventory. Phalanx doesn't care what is out there it automatically fires on everything that approaches the ship at virtually any speed and small size which is why it is turned off in port and in many situations. (You can't recover your own helicopter with it on.)
I am not saying that you can always avoid missiles but it is certainly much easier than most people would envision especially when you are just as agile as the missile. Which is why IMTU I assume missiles are not limited to 6g accelleration and are so quick that they impact the same turn they launch at reasonable combat ranges. (Traveller starship combat turns being so long it makes sense to me.) Besides then I am not bothered, especially in a fleet engagement, with keeping track of all those missiles. (And a fleet engagement doesn't neccessarily involve Capital ships in this sense. 3 800T mercenary cruisers vs. 4 400T SDBs can put quite a few missile salvos up in three turns.) Forget about trying to keep track of the missiles of one Atlantic class or AHL Cruiser.
One other point on missiles. In most situations you are not going to see missiles as effective KE (Kinetic Energy) weapons, mostly it will be glancing hits, proximity detonations, not the hit the ship with a c fractional velocity crowbar that will rip it apart type strikes. Relative velocity of the missile to target will matter little in space combat once you actually generate the hit. Though the hypervelocity direct hit makes good reasoning behind the traget destroyed Critical hit result.
Originally posted by Hal:
Much of what I'm envisioning doesn't seem to match your description of missile combat. I'm envisioning an environment that is based on standard physics where the combatants are approaching at say - 5,300 miles per second. The ships get one pass and that is it. The missile gets one shot at achieving an impact and that is it. A 6-G missile that operates for one hour is only going to be able to change its velocity by 131 miles per second in that one hour. A 10 G missile can only change its velocity by 219 miles per second in that one hour. There is no way that a missile that misses is going to be able to slow down, turn around, and come back at you with anything near the original 5,300 miles per second speed you've worked up to (that's assuming that you were accelerating for 2 days, 12 hours and 40 minutes at 4 G's. 
Using the laws of physics and such for vector movement - displacement is equal to one half acceleration times time squared plus your initial velocity times time. The computers can plot based on your known velocity at the moment you are observed and determine just how much you can alter the target point (ie where you ship will go) by only 1/2 a * T^2 for any given measurable time period. You can slow down (thrust against the direction you are moving), retain the steady velocy you've already built up without any changes (ie add nothing to the current built up velocity) or you can change your velocity by up to some value between zero and max speed.
If your maximum speed is 4.4 G's, the maximum displacement your ship can achieve in 20 minutes (one turn) is 17,542 miles centered at the point of where your ship would have ended up had you NOT done anything to alter your current velocity and vector/bearing of your ship.
I hope I'm making sense here...
