Water-resistant panniers have served me nicely for the kinds of touring I do: mostly day trips, an occasional weekend outing, and an infrequent one to two week tour.
I find water resistant panniers with waterproof covers are fine when it's drizzling. During heavy downpours, I seek shelter and wait for the weather to improve. I have been drenched on rare occasions, and the stuff inside my panniers got damp -- but not soaked. I hung everything on a line overnight, and by morning everything was dry enough.
I would consider waterproof panniers if I were to embark on regular, long tours. But for the shorter jaunts I currently do, water resistance works for me.