Most guys aren't physically capable of being 240+ and being effective though (naturally). You need a certain and very unique body frame to pull that off. If you're 6'0 and try to get to 250 of pure muscle, that shit aint gonna work out well.
And we've seen this over and over against the Klitschko's. Some guys come in light (from 205-215) some guys come in heavy. The problem is none of these guys have been genetically gifted like the klitschko's, so they are either just too damn small, or they gain too much weight which makes them too slow and/or too fat, which in turn can make them in poor conditioning, meaning they'll run out of gas after 4-5 rounds.
Anyone can be 240 if they want too, it's just a matter of how effective they will be, like you said. That's why you can't really put a cap at weights like that, because then you would have to take things like genetics into effect, which wouldnt work.
There is a guy at my gym who is 5'6 240 (muscle, not natural of course) and he looks like a rollie pollie, that wouldnt work at all.
People come in heavier than the brothers and they still can't beat either Klitschko. It all comes down to genetics (height, reach, etc) and skill.
Making a seperate wieght group at that level wouldn't do anything but start a freak show. All my opinion of course.
You have fighters like Brock, the Klitschko's, etc. and that is what makes it entertaining. Who can slay the giant type deal. Plus I throroughly enjoy watching either Klitschko brother batter their opponents into a pulp. I'm a sick person I guess.