*Membership spots not really limited!
I dunno, I think there's plenty of variety when it comes to in-ring styles in WWE.
What NXT does best is allow people time to develop characters and gimmicks. Bray Wyatt initially debuted as the boring 'Husky Harris' in the WWE, but he went back down to NXT and was allowed to create a character that has become hugely popular.
I have no doubt that El Generico could have been dropped straight into Raw and do well in the ring, but being in NXT let him take his mask off and learn how to do promos properly rather than speak pidgin spanish in an environment that's less pressured than 15,000 baying fans.
WWE is about more than just in-ring style, it's about character, and that's what NXT is for.
There's way more of a variety of styles when I watch New Japan though. I think Syxx is right in this case. The Fed does promote a framework that can work for some people. The problem is that it doesn't work for others, and I've found, for my part, that it really doesn't work that well for me as a viewer.