I don't think it plays that huge of a role. I think it just legitimatizes or explains the score you got in a class. For example, if you got an A+ in a class, but got a 2 on the AP, that sends up a flag about the real difficulty of your course load, whereas a B- with a 5 might explain the "low" grade. But then there's also the argument that some people just have on or off days and that one test can't truly gauge the level of a class.
Let me give you some true life examples!
English Language, 97 UW average, 3 on the AP
Euro, 95 (highest score!), 4
APUSH, 99, 5
Stats, 99, 4
Chem, 96, 4
My scores don't really correlate with my grades, but I still got into my top 2 college choices.