Wise and Insightful
Not for me. If primes were just bonuses, then the DC would always be the same number. To me, it's easier to have one variable rather than two.It's easier to go "roll, add 4, compare to 12" mentally than "roll, add 4, add 6, compare to 18"?
EDIT: To be honest, I'd be happy to go the other way and modify the DC only, so I just have to roll the d20 and get a 7 or better, or whatever.