In a real military, there is a WIDE VARIETY of different uniforms in use at any one time. It would be interesting to see this variety. The only time we saw anything remotely like this was in Generations.
The US Army has 3 standard uniforms. The A (uniform with jacket), B (uniform, no jacket) and the ACU (camoflage)
Then you have assorted jump and flight suits. And it isn't as if you can pick whichever uniform you want for the day. Base commanders usually decide which uniform will be worn, and that depends on the type of work involved. Most bases go with the camos while the Pentagon - along with more clerical personnel - goes with either the A or B depending on the weather.
And this is why I love Star Trek's vision of the future so much--in the sense that it separates itself from today's military by being more lax, while still having a strong chain of command. These are exploratory vessels (for the most part). This is humanity. This isn't all brute force and explosions. Starfleet has to show the rest of the universe what humanity is like, individuality and all, and the military of Trek's future exemplifies that ideal.