Actually, that's not why 3do based terrain is used. Even my first experiments with 3do terrain was using segments that could be culled in the distance using the first RVTCS. The framerate benifits, not having to worry about the adjoin limit, and being able to make more complex terrain was a bonus.
The only real advantage I can see is being able to use surface triggers in cogs for specific places on the terrain. However you loose the use of dynamic lights throwing realistic day-night transitions completely out the window.
Other then the use of surface triggers (which you can ussually get around by determining the xyz coord over the intended 3do surface), what other costs are there? (and yes, 3do terrain can be lit quite nicely with dynamic lights....)
http://forums.massassi.net/vb/showthread.php?s=&threadid=17347
http://forums.massassi.net/vb/showthread.php?s=&threadid=17321
It does indeed open the doors for using sectors to do outdoor terrain. It's certainly a viable option now. I just don't think that I would use it. But that's just my opinion...
I'm still holding out for a break in the apparent sector size limit...
Math is infinitely finite, while the universe is finitely infinite. PI = QED