Ah, there's already a :nextval. Now I see your point. I guess someone (possibly me) thought that was a good idea at the time. My issue is, though, that there's so much built-in functions in Postgres that providing a convenience wrapper for all of them would make the library huge.
I'd remove nextval, but that might break someone's code. The consistency isn't too bad, since the definition of :nextval also allows strings as input, people passing these functions strings (the logical thing to do) will never even notice this.
Best, Marijn