Dex improves your proc per minute (not per swing)
Dex does improve your chance to crit (ranger with bow or warrior otherwise)
Str affects both your global attack (chance to hit and hit harder on the DI 1-20 roll) along with modifiers to how much damage you do
Regarding melee dps ... warriors are the only ones who should functionally care about *dexterity* for the proc rate. There are exceptions for rangers with BFG burning trueshot on a named. At that level of gameplay, however, both dex and str are easy to cap with buffs. In general a ranger should care more about str than dex on p99. Even in the case of root/rot/bow kiting, str probably pays off more in the long run.
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