I perform live almost every week. And part of my keyboard rig is a mainstage rig. Part of playing on stage is reliability - you want to make sure youve got enough processing power headroom just in case the thing spikes, and your synth pad suddenly stops playing and boom. Your set is now awkward. The the i5 processor is capable of running most plugins on logic pro and main stage, as well as a lot of the third party plug ins that I use. The i7, would be able to do all that with a lot of headroom so I don't get the stupid CPU overload message that kills all sound. So thats the justification for the i7
What about the justification for the air? I currently use a quad core MacBook Pro - just because I also use it for more intense stuff - ie some production/recording. But I can see why you would use an air for live performance - space and weight savings. When you're hauling a trailer load of stuff, the last thing you need is another 10kg backpack that you'll need to carry the pro. The air can actually fit between the foam of a guitar pedal suitcase, one less thing to worry about carrying. May not seem like much, but trust me.. When you're touring, constantly worrying about carrying extra crap as to not lose it gets old real quickly.
Secondly, it comes with SSD. In a live setting, things bump, vibrate and drop. The reliability of an ssd, at the price that it is, is a good good bonus.
Boom.