While it is correct that jumps like this are calculated, and so hypothetically pose little risk, that's never actually the case. The mere presence of wind that is different than planned for can have disastrous consequences, not to mention what mechanical failures can cause.
I used to race motocross, and it's awfully fun going 100 ft in the air over big doubles and triples, but I am still amazed at what the freestyle guys are doing while in the air (backflips, superman, etc). Either way, I can't say that I'd be remotely comfortable jumping up to the roof of a building, and even more so, coming back down!