That's a good point about the ability to change uniform insignia with tasking. However, your original department, ID, and rank would still be visible usually. Remember, in a space suit, it is hard to see epaulets, wrist-rings, or collar dogs.
Plus, in busy environments like a flight deck (reference modern US nuclear carriers), colour coding can be very useful. It identifies who is on which teams, has which jobs, and lets a senior officer overlooking the deck immediately see that people are in the right or wrong placements for the type of operation being executed.
As to the comments about MOPP4, I'll point out:
1. Rarely do your suits have air conditioning. Vacc suits may well feature quite advanced climate control, thus easing the strain (one sure prespires in those lovely charcoal lined suits...).
2. Modern gel linings and uberfabrics may well make them a lot more comfortable to wear in and less likely to chaffe.
3. Having said the last 2, I still agree. The longer you are stood-to in full spacefairing gear, the more fatigued you will be. This goes doubly so for lower tech bulkier gear (hence another odd and minor but notable advantage for higher tech forces with tailored suits) and for guys in battle dress. Marines especially will have grown used to the long periods of being encased... but everyone is always glad to get out of the suit, I'm thinking.