Dont you want to do most 8-man-content with a premade group anyways? So if you feel confident enough, you can go in with only one tank.

I realize that NM of raids and primals are exceptions here, but for the first I'd really prefer a back-up tank (aswell as the fights sometimes having mechanics that require tank-swaps after all) and the later are normally run once and then never again (and, again: tank-swaps, back up tanks)

And as cheap as a mechanic a tank-swap might seem... as a tank myself, I like it - it keeps me on my toes a bit, gives me something to watch out for and to coordinate about. As MT many fights are "stand still in this spot and get your ass wiped" already - tank-swaps or switching to dps-mode are a nice change from that.