We used one a bit when DS was young and very unsettled, but ony ocasionally. As he got more settled, we didn't use it so much. One day it'd been nearly a week since last use, I tried he would'nt have it. end of dummy.
I think they can be useful things. If breastfeeding is well established, then there's less of a risk there. There are 2 possible worries in regards to bf, btw, not just the sucking action, but by satisfying the sucking reflex away from the breast they may not feed enough. If she's feeding well and gaining weight etc, then it's all good.

My only real issue with dummies is that they are unattractive

ETA - the thumb/finger sucking thing is pretty random also. DS sucked his fingers till 9-ish months, but never since. It's not a given that they'll do one or the other.