Larger drives mean you don't have to have so many in a machine, all drawing power and making heat and noise. So there is a point to it, and it makes sense. Plus, it's cheaper buying one large drive than many smaller ones, then buggering about with the RAID setup, and all that overhead.
As for the spindle speed, at that density you're looking at quite a lot of data passing under the head rather rapidly, so it makes less of an impact than you'd imagine. Coupled with a nice big buffer and plenty of read-ahead, even less again. And a slower spindle means less power used to get it to operational speed, so less draw on your battery. Win, game, set and match.