Style, man, style!
It's true, it could be done, and I'll probably end up building a deck up there anyhow, which would further lend weight to the idea.
But not only am I afraid of scraping them off somehow (Whoops! Did you just hear something?!), but I also want her to look like a clean machine, and half a dozen bikes tied down on the roof kind of defeats that. Plus, think about the mileage drop
I'm also thinking about other gear that I might not want to put on the roof, but who knows, I've never loaded it up, I might not know what to do with 30 feet of bus (not counting the 5' in front for driver/nav).
So I'm kind of thinking bikes on roof as last resort, but an additional bennie to the liftable bed is that the rear door is still a viable loading/unloading point for large items. Not so with a bed mounted there.
So goes my (convoluted) thinking.