I can certainly see Gotham Central's point here, though. It wasn't easy to distinguish ones' position on the ship relative to another. We knew who the characters were, of course, so it was sort of secondary, but still: Kirk at one point wore the pale blue shirt with the long sleeves, then at one point switched over to the short-sleeved white shirt with the v style neck. But Sulu also wore that style, if I remember right. Then there were the pale brown shirts which I thought Scotty wore, but I'm realizing now that he probably didn't. I don't remember who did. Spock's uniform was the only one with the dark raised collar. It did all come across as a bit of a jumble...I do wonder if it would have been confusing if they had had an entirely unfamiliar cast and crew.
According to background info in the old TMP "Make Your Own Costume" book, which had input from Robert Fletcher, personnel were allowed considerable leeway in their uniforms. The different cuts, colors and configurations gave more individual options. Spock's standing collar and McCoy's v-neck/spread collar were their own personalized touches.
As far as being confusing, that shouldn't be a problem within the crew, as long as the rank and department color are visible. If you want to take a "naval" model for Starfleet, navies have traditionally been more relaxed about everyone looking the same, standing in straight lines, marching in step and all of that. For instance, this picture from an aircraft carrier:
All things said, I prefer the cloth patches and uncomplicated design of The Motion Picture uniforms. The 'away' jackets from The Wrath of Khan are interesting to look at, but just imagine on a planet, a young, action-packed crew crawling around and running through brush with those things. With all the adornments, loops and hooks getting snagged on stuff. They just don't have the dust-yourself-off-and-keep-going utility of the the 1st movie suits, much like the originals from the series.
In TMP the metal pin breast badge as worn on Kirk's first uniform was supposed to be for "ground" wear, while those of shipboard personnel had theirs sewn on so it wouldn't snag or poke in the close confines of the ship. Those considerations were obsolete by TWOK, apparently, when the duty uniform had a surfeit of pins, buckles, straps etc.
I tend to agree. In contrast to most people, I don't mind the pastel look of the TMP uniforms and find them much more realistic than the TWOK British Redcoat attire. The FC uniforms remain my favorite in all of Star Trek. But the TMP uniforms are a close second.
I understand that they wanted to wipe the slate clean for TWOK, but my preference would have been for the new uniforms to look more like something that could have plausibly evolved from TOS/TMP, rather than such a radical departure from what had been established before.
One additional item I've been wondering about: did Kirk take Decker's quarters on Deck 5? Or was Decker already staying somewhere else? We never see Decker's quarters- or anyone else's for that matter, save for the crew quarters in which the Ilia probe appears- so there's no way of knowing where he was when off-duty.
It is possible that there were comparable quarters set aside for embarked VIPs or flag officers and Kirk could take those without displacing Decker immediately.