What I really like about the 645 format is the 3:4 aspect ratio. If you take an 8x10 piece of paper, and make a 1" border around the entire print, you get a 6x8" print area, which is a perfect 3:4 aspect ratio. Plus, the print looks fantastic in your hands.
Same goes for 11x14, where if you take 1" border around the print area, you end up with a 9x12" print, also perfect 3:4 ratio.
It almost works for 16x20 too, but in order to stay true to the aspect ratio, you'd have to make a 13.5x18" print, which leaves a 1.25" border on the long side instead of 1" as with the other two.
If you really dislike 'wasting' paper, you could always import 12x16" paper from Europe, a common size there.