Because people assume ARM would give us better performance and/or battery life. Hard to know if that would actually pan out in the laptop form factor (and if it would hold true forever), but at least right now iPads get better Geekbench scores than MacBook Air.

As for more money and time… I dunno, I make my living writing mostly iOS and Mac software and I can't recall the last time I did anything that was architecture specific. I'd imagine that applies to the vast majority of people working on ordinary application software on these platforms.