An example of the opposite situation (Opeth's one) has just come to my mind: Eldritch. The first three album were made with a guitar and a keyboard, but I think that they really weren't able to combine such elements in an effective way. When the keyboardist quitted they took one more guitarist instead, and their sound has come to be much better now (in my opinion)... it is more compact and straight, much less progressive oriented and some more thrashy.
In general, I do like keyboards, though it always depends on how they are used (expecially along with the guitars).