Remember, gear is not there for looks. It serves a function. It's very, VERY easy to overdo it, and a lot of noobs do. Hell, even some more seasoned "geardos" often show up with FAR more shit than is actually required.
For small skirmishes, you can show up wearing whatever you want as long as you have your eye protection. But in terms of actual milsims where there's a level of roleplaying involved, teams are separated by uniform types. You either follow the rules of the event and fall into a team with the proper colour / pattern, or you don't play.
When it comes to vests and such, they're kind of needed since you need to carry mags, water, dump pouch, and other accessories around with you on the field. The vests or rigs used for that are not for cosmetic purposes. They're actually quite functional and required.