I've mounted my Marshall 70XP on my 7D and it's been fine. Not only that but I've used the redrock microHandle Plus (http://www.evsonline.com/redrock-micro-microhandle-plus.html) to do plenty of low mode shots, so it for sure can support at very least the weight of itself with a 24-105 on it. Would I put a 70-200 on it? Probably not.
Also, its very important to know which camera we are talking about here. The 5D and 7D are the only ones i can feel safe about really mounting anything to the hot shoe because of it's single cast magnesium body which the hot shoe is drilled directly into. On the t2i, it's basically a bunch of plastic peices snapped together, and the MOST I'de but on that is the marshall 5" if I'm not swinging it around.