Mostly it's just a trophy to unlock and a bragging right. You get a symbol next to your name in MP. More head slot selections after reset, and again at 25. What I like tho is the class challenges reset too, so you can still get extra XP from completing those.
I hit 25 on all classes yesterday, but I've yet to reset them for Prime Mode. I'm doing so well with my current weapon/ability/upgrade choices, I don't want to get rid of them yet. I'm waiting for a EXP multiplier weekend (hopefully this weekend) to reset so I could quickly get back my upgrades. Those insecticon heads look so tempting, but at the same time they remind me of the week-long grind it took me to get to 25 in the first place.
