Keep in mind, too, that at the time everyone was being told that TNG was not going to last. The actors were signed to 5-year contracts because that is a standard in television, but were all told "don't worry, this thing won't make it past the first year." And, at the stage in the game they were when Crosby left, I can understand why she might have thought that.
Plus, as others have pointed out, the writing for her character -was- terrible. She had great potential, but it was never used. When she was used, it was written so badly it was painful, especially the dialogue she was given.
So, yeah, in retrospect it was a bad move for her to leave. And if I were a struggling actor, I'd probably hang on to a steady series gig as long as I could unless there was a -real- good reason to leave. But that's just me. Other actors feel differently. Her character was being badly written and underused, and everyone thought the show wasn't going to last. I can't really blame her for her decision.