During my flight training I flew with quite a few instructors, at quite a number of different schools (in 3 countries), and a number of different aircraft types as well. Each instructor taught me something that the others didn't and the experience I gained from all the changes, I believe, made me a more competent and experienced pilot. I wouldn't change it if I had my time again.
I agree it's good to stick with one instructor at the early stages of training where continuity is more important, but how do you know how good an instructor is without comparing them to others? My opinion.