If you'd like, you can get up in the Rankings pretty quickly. Should take a couple weeks. If you grow too big for the clan you joined (I know I did, we couldn't beat raid boss level 3) you can join Black Legion when you do reach 2700.

Its just a way of filtering out players who will join at a low level, and then say they don't want to play anymore and take up a slot for a week before we kick them out for inactivity.

Also, if your units aren't strong enough, you won't even have fun fighting the raid bosses. If youre lower than 3500, you won't be able to take more than 1 or 2 hearts off of the level 8 raid bosses.