One stat to rule them all, or what if we merged Dex and Str?

[Willie the Duck]

Is there something I'm missing that would be grossly overpowered with that change?

I don't know about overpowered (since I've seen wizards with a random 14-16 str before and it didn't make them a melee powerhouse), but what happens to the non-gish wizard that previously had a decent Dex for defense? Do they now have a decent Physique (and, if so, what differentiates them from the gish)? From the standpoint of using attributes to capture thematic differences in the character you are playing, I think this reduces the granularity.