Well, if they're brain dead, they aren't suffering, but if they can't enjoy life or make use of themselves, why shouldn't it be allowed.
It's just how you do it that should be regulated. Example....Terry Shiavo (sp?). They decided to let her die, so the disabled her food supply and she died.......two weeks later! That's messed up. If you think it'd be best if they died, you'd think they would want it to be quick.