Want high ISO performance?
You're already on a tiny sensor, do you even use all 18 mega-pixels? How much do you crop when you're already on a crop body?
If you're pictures only go on the internet, you might as well have a 12/14 MP sensor, and still be scaling them down massively anyway, I wouldn't personally see a point on going over 15MP for a crop, if that, I mean you need 25+ for a billboard, and who would shoot a billboard on a crop?
TL;DR: More MP on crop = unnecessary and possibly counter intrusive for real use.