As I just mentioned on another thread (before seeing this one) I just bought an old Land Cruiser (mainly for exploring) and will very likely buy a RTT for it. I think they fill a great niche.
Of course I still plan to find the perfect TC someday, but for now it'd be ideal for the kind of camping I'll do the next couple of years.
When my boys (19mo, 1.5mo) get older and camp with me I'll do it in a TC or TT, but for quick 1-night solo hiking or mt biking trips a RTT would suit me just fine.
In my mind they "split the difference" between a tent & a TC.
Advantages over a tent: no finding a level spot, clearing it of debris, trying to somehow keep the tent clean upon multiple setups/takedowns, and you sleep off the ground.
Advantages over a TC (to state the obvious): no need for a HD pickup, weight/size, initial cost, very little cost to maintain.
Now, is it as comfortable as a TC with a full bath, heat, fridge, etc? Of course not, but everything is a compromise.