I like the idea of the rooftop "shelf" where you can put anything you want, as long as you're ok pitching a tent up there (or pitching on the ground and hucking it up) it sounds like a very good solution.
I do think tho that the difference between a "standard" rooftop tent and pitching a normal ground tent on a roof rack is ease of pitching and stowing, and comfort. Personally I'd be more inclined to just leave a ground tent on the ground rather than try to pitch it or move it on top of the vehicle. I do see a benefit tho in that 3-season, you can put an air mattress up there, get off rough terrain, get a better view, and probably sleep more comfortably...plus you're guaranteed a place to tie it down, extend the fly and whatnot.
That Oasis II is really nice! But is it 3-season or 4-season? Anyone know?