A lot of people laugh at this idea but to me it makes sense and at the very least at least you have two backup devices! Mobile phones can be added to the equation as Kaos replacements - I don't see it as very cost effective though, I'm keeping my money for the VMS4 ;)
One drawback with using an iPad (or any touch sensitive tablet) is that scratching is tricky due to the friction of rubbing a piece of glass. And for the record (no pun intended) he's only partially using the iPads to DJ with because the iPads are timecode controlling Serato so the music is still coming from an outside source. Apple won't allow you to use music contained on the iPad (unless maybe you have a jailbroken iPad).
Controling the pads thru timecodes is good enough for me. Even if i was allowed to ad songs on the pad, i woun't. Its a longer process to search for songs and switching back to the deck screen. Imagine doing this all night... No way. The decks running in time codes is good enough and i will trigger the cue points on the laptop keyboard. Looking for songs has to be quick.