Smart people work smarter, not harder :p
When I do an onion, I cut the ends and remove the skin, THEN throw it in the chopper. No mess. It's not that I CAN'T chop it with a knife, it's that if I want it finely chopped, the chopper thing is much, much faster. With three kids and everything else I've got going on, if I can get it done faster, all the better when it comes to cooking.