Well, I think its just more of that idea that it could be easily abused. Not because it looks messy. Which is used properly it can look really nice. But when you have things like look like this..
ThIs iS MY sIg!!1eleven!
Well, thats no good. But something thats like..
This is my signature
Now, that looks decent, but when you have people doing things Like:
tHIs and ThIs with weird colors, and numb3rs. It just gets out of hand and plainly looks like crap.