My US$ 0.02 on level 0 weapon skills:
1. I like the idea. Let's face it, adventurers in movies and books use all kinds of weapons with no evidence that they learned them before the current encounter. Captain Kirk outfenced a musketeer (and he and his officers did pretty well with swords against trained Klingons). Luke Skywalker outshot trained imperial stormtroopers, with the stormtroopers' own weapons (as did Princess Leia). I see no reason to assume that adventurers wouldn't spend a little time learning the basics of how to use weapons.
2. In a 2d6 system like CT, a skill level of 3 is huge. Effectively, you'll hit most of the time (and just about *all* the time if you have the advantageous DEX). So to keep PCs from being able to effectively hit all the time with large categories of weapons (i.e., all pistols), I think that Marc Miller compensated by limiting weapons skills to specific classes of weapon - an Autopistol skill does not work with Revolver, for instance. As a game balancing tool, it's a reasonable choice. However, it is a bit unrealistic. So giving everyone a skill level of 0 mitigates the worst effects of this approach. (As an aside, the reasonably well considered CT system was wrecked by the weapons, skills, and character generation system that appeared in Book 4).
3. I think some folks overestimate the difficulty of attaining a basic degree of competency with firearms. To quote James Dunnigan, "With a $400 dollar .223caliber, bolt action rifle (and scope), a shooter with a few hundred rounds of practice shots should be able to his the target 90 percent of the time, which is what the DC sniper has done. ..." Seems likely to me that most adventurers will spend some time target shooting and it isn't unreasonable to give them a level 0 in most firearms. If you want to be pedantic, you could give them a level 0 in all firearms *if* they have a level 1+ in any firearm skill.
4. Paradoxically, I think most folks underestimate the amount of training required to use a firearm *well* in a combat situation. The Dunnigan quote above refers to a trained sniper. In CT, a person with Rifle-0 and a non-evading, unarmored target will hit 84% of the time at medium range. However, in non-sniper adversarial combat situations, the average to hit rate is far less, even with trained shooters. Unfortunately, there is no easy fix for this problem in a 2d6 system unless you're willing to ruthlessly pare down to hit modifiers. Plus, it's defensible as "dramatic license". It's the same reason no one drives around looking for a parking space in movies...it's boring and adds nothing to the plot.
5. Melee weapons *are* much harder to learn to use compared with guns, but an automatic level 0 is still okay with me for the simple reason that CT allows the opponent's skill as a negative DM. Someone with Halberd-0 is only going to be very dangerous to someone with no melee weapon skill (or no melee weapon). Against a trained opponent (skill level 1), he'll hit on a 9+ (28%) and be hit 58% of the time. Against a highly skilled opponent (skill level 2) he hits 17% of the time and is hit 72% of the time. Against an expert (skill level 3) he hits 8% of the time and is hit 83% of the time. And again, someone who chooses a life of adventure might try to pick up some useful fighting skills.