I have two backup drives (small, bus powered ones). I use one with Time Machine and the other with Super Duper! My reasoning is that they each do slightly different (complementary) things, and plus if one of the backup drives dies, I have the other.
I may be wrong (not an expert), but my envisioned use for the Super Duper! drive is my computer dies and I can then plug it into someone else's computer (or a loaner) and it will become "my computer," running off the external drive.
I basically use Time Machine to make moving to a new computer easy, but there's always the chance I'll use it for something else.