It's not a niche thing at all; I know plenty of people who have brought macs for things like iMovie (Including myself) iPhoto and lets face it, running itunes, instant messangers, a DVD maybe, and some other things does take processing power. As the OS gets more and more lines of code and more complex GUI, processing power becomes a premium. Plus, things like making music (logic in particular) demands a very, very high processor ability, lots of ram etc.
I think you're being to personal about YOUR processing needs. Don't forget the Powerbook is a "Pro" line. People willing to pay $2000 for a laptop don't want a ridiculous 1.6GHz. I can't even run 10 plugins on that. Poor.
Hence the move to intel of course. I'm all for it.