Over commitment is not always a good thing. When you accept more than you are capable of realistically completing, you begin to feel stressed and anxious because you are in over your head. You also may not be able to follow-thru on your commitment. Balance and the ability to understand your limitations is important to success.
Too much commitment is a good thing. Usually if you are devoted and committed to something, whatever that may be, usually means it can grow and thrive. If you are driven and passionate about something, then I say go for it and see where the road takes you. There is never too much commitment I think.