The bugs just make it more fun
Transformice proves that if you have a lot of people together in a situation that requires teamwork, something will probably go wrong. Fortunately, this makes the game more fun. The simple graphics and controls make it easy to pick up and play, and there's plenty of advanced techniques like wall jumping that you can learn to give the game longevity. The increasing population of the game as a whole to the point of 10 servers including this one is well justified.