Actually, that reason is both valid and common for thumbing things down. Personally, I probably thumb 1/25 of the comments I read, but if I didn't get it and decided to thumb it, it would be down.
Me? I only thumb things up, never down. I would hate to be responsible for a buried comment just because I didn't understand it but someone else would. Ex: this comment :P and I don't worry about overly offensive topics. Those usually get mod anyways.
Obviously.