In my experience a left side (ignition side) leak will cause your bike to run lean. If its bad it can run very lean and potentially seize. If you take the flywheel and ignition off you can sometimes see it leaking oil. Some bikes you can replace it from the outside, some you have to replace from the inside which sucks cause you have to split the cases to do it.
A right seal leak I've never had, but I understand that this is a wet seal between the crankacase and the gearbox. So when it leaks the bike sucks in oil which causes the bike to run rich - I think.
There's a good thread or article on eric gorr's site about this. I was just reading it, but don't remember exactly where.