No the components that make up your computer are far from irrelevant.
But if you're programming on the Mac then it doesn't matter what CPU you're developing for because XCode handles it more or less transparently. Likewise, if you're a Mac user then it doesn't matter what CPU you use - you'll either be using a dual-binary build or an emulated PPC build, but it's completely transparent to the user.
Except not everyone is "working with XCode". Like people who develop games for instance. Like Blizzard.
Of course they are, and obviously there'll be dual binaries for a high-profile Mac game.