It may be that I'm just new and am missing something, but I'm pretty sure that at some point if you had an item with high enough QD it would outperform straight STR or DEX.

there aren't lots of items around

Haha fair enough. I've been using an 18% QD chain mail lately and it boosts my damage by a shit ton, especially on double/triple/quadruple procs.

QD a great sidemod, but not that usefull main mod .. like power shot, ( if using high range wpn types like a long sword )

Yeah I think that using one item with a high QD % or 2-3 items with midrange QD% would be the best way to go, but like Benell said the items are pretty rare. I'm not really willing to do spreadsheets or anything crazy like that to test it but I definitely think it could boost your damage more than mass STR at some point.