Well, it’s lot cheaper, for one. It’s also a lot smaller, more compact, and lighter to carry to the gig.
The software for TouchOSC isn’t as sophisticated as that for the Lemur, but it is remarkable given the price.
There is some latency with the wireless connection, which I only noticed during the performance when the performer next to me was using the same setup (Macbook + iPad) On a different wireless channel.
Lastly, the iPad is a general purpose tool – I’m writing this on one…