I honestly think it makes things way more complicated. Leaving other software and websites out of it, SMF has always had "register", so first of all you aren't making it "friendlier" for SMFers. Can you see the support questions rolling in. How do I register? Where did register go? Then, adding in other software and websites, where recently on big sites have you seen "Sign up"? Which isn't necessarily a reason NOT to do it? To be like other people? *HOWEVER*, the argument is, that's what people are used to seeing. They understand the terminology, and what applies, etc.
I think that changing the linguistics on something so basic and elementary is not smart. But good luck...