Except that makes you far less likely to get a good stat where it matters most. A Wizard, for example, could easily be stuck with an Int of 14. Power wise, he'd be equivalent to any other Int 14 Wizard, most likely. The fact that he might have a Wis or Str of 16 hardly matters much. Honestly I'd imagine going the other way would be better... through an 18 in your primary stats, and hope the rolling removes any 8s you leave in your build.
JaronK