It never gives me enough time to fix mistakes in my post. When you write something longer than a paragraph, "mistakes" aren't mere typos. Besides, I often walk away and read my comment again after some time has passed, and notice an error or omission that I would have never caught obsessing over it for 10 minutes.

Such a short edit window will discourage lengthy posts on myUMBC, lowering its quality.

EDIT: The 10 minute edit limit, by the way, is the thing that turns the "edit" button into a "locked" button 10 minutes after posting a comment. Try it yourself if you don't believe me. :-P