3do based terrain is used because sector terrain quickly hits the adjoin limit and also creates a signifigant framerate hit for more complex terrain.
This does opens the door for it to be possible to make standard sector terrain, with all of its advantages (better lighting, vastly increased potential for manupulation of surfaces, etc) However, there's still framerate issues which may or may not be a concern for some people.
3do terrain offers good frame rates at the cost of some features of the JK engine. (Can 3do terrain be lit decently by dynamic lights?)
The important thing is, new doors in editing are opened. For simple outdoor areas, I'd continue to use sectors. For large, complex outdoor areas I'd continue to use 3dos for framerate reasons. But for moderately complex out areas where once 3dos were the only option, I might consider using sectors now.
-El Scorcho
"Its dodgeball time!" -Stormy Waters