I read once that the idea was to give the futuristic starships a comfortable, "shirtsleeve" environment.
And anyone notice that JJ's medical uniforms look just like the TMP medical outfits. (And The Admirals uniforms, of course, look obviously inspired by Kirk's from TMP...)
One idea I liked was that there were *off-duty* shipboard outfits too (look at the bath robe-looking outfits...they were meant for off-duty I originally read in an old "How To Make Your Own Star Trek Uniform" book...)
I mean, as much as I like the ST:2 and beyond "band uniforms" the look uncomfortable to wear all day. They look like a *formal* uniform (and *double as such*) and we needed a ship-board uniform to go with it. Maybe a landing party outfit too.
I like the "bio-belts" and just imagine that they were later miniaturized and are still part of the uniforms somewhere. I thought Star Trek '09 shows that it made sense to be able to monitor crew's lifesigns off-ship. (Only think missing is a camera. We have them *now*!)
I would love to see some version of the TMP uniforms with maybe the TOS colors (if maybe a bit subdued) - and *maybe* black pants...just to see how they look...(but my Photoshop skill don't exist.)