Double Uppercut: not so interesting?

I was comparing the 1 DU > 2 FF > 1 DU comparison--Not the Backspin comparison.
Of course Backspin would be better overall then DU due to mobs. But when mobs aren't around--well, you get the idea.

You were talking about what DU adds to FF, I was talking about what Backspin adds to... well, itself. Compared to it, maxing DU doesn't give much of a benefit even in the context of single attacking.