Using the margin to pick up some beaten down names here & there that I think will grow nicely. Or simply avoid selling stuff to spend (I'm waiting for BRK.B to make a move again before selling my big position there).
The calculus was totally different when I was paying 12% - at that rate, I wanted to close up any margin balance ASAP.
And I can use Form 4952 to either put it towards cap-gains, or save it to put it towards future gains. I could even use it to knock down dividends.