The uniforms do seem to sport pocketlike attributes when needed. A seemingly pocket-free TNG uniform can suddenly sprout a holster for a phaser and a tricorder, for example.
We could assume that the apparently seamless fabric of these uniforms is designed to open and stretch in a suitable manner when a pocket is needed - much like a uniform jacket opens seamlessly when needed, as we see in "Ensign Ro".
That's something I could easily see as advantageous: pockets that are there only when one needs them. The rest of the time, they don't chafe on your thighs or gather strange yellow-grey goo or toothsticks.
Of course, this magic fabric probably isn't a completely perfected product in the 24th century yet. We have seen this fabric rip under stress. So for storing big, heavy or sharp objects, a somewhat more conventional pocketed harness or vest is still practical, as seen on our engineers every now and then. But give it a hundred more years... Eventually, all items such as tools might be carried on a seamless costume seamlessly, requiring no pockets, loops or holsters, and there would be no need for things such as belts, belt buckles, shoes, shoelaces and so forth. The costumes would become unsoilable, thermally ideal, perhaps phaser-proof as well. That's simply something that's just beyond the horizon in TNG, or perhaps already peeking a little.
Timo Saloniemi