I was wondering, why does people count dps instead of total potency?

[Inuk9]

the biggest mistake you're making is believing crits are random and thus cannot be counted on. Over more than one parse crits are no more random than main stat, weapon damage, det or anything else.

This.

Potency per second is usefull when using in theorycrafting, so we can learn best rotations. But DPS is the final result of combined rotation + stats(including crits). It's the ultimate result that tells you if you build your character correctly and learned proper rotations.