It seems to me that quite often you're only going to get one crack at a fleeting enemy. Even in more open field fighting vagaries of terrain and relative position may only give one guy in the team a shot at a given opponent. That shot had better be fast, accurate, and effective.

PDWs are fast, but they aren't as accurate as rifles and their rounds can't be as effective.

The extra heavy weapons a PDW equipped squad can carry are effective and accurate, but they aren't nearly as fast as a rifle.