I do from time to time. If you look through the Drastic DS reviews you'll see mine eventually. Capcom wouldn't bring phoenix wright to android in english even though it was out in japan? Well ha ha, I have phoenix wright on my phone anyway.
It is totally overpriced, but when you consider the jump from every other ds emulator to drastic you (i) mentally knock off a few bucks anyway.
I also bought fpse, and epsxe, but other than that I use free ones like gensoid and nesoid.
I have to disagree with the "illegal games" argument though. I own a LOT of cartridges for nes snes and genesis. I'd say I own about 99% of the games I'd ever want to emulate. When they're disc based, I usually even rip them myself.
Having said that, I know that downloading a game you own is still technically illegal if you didn't rip it yourself, but I don't find it morally wrong. I just don't have the hardware to do it to my carts.