So for less money, I get access to the full range of software - why the hell go PPC? Fanless? That's all?

This is only half of the picture. It's fanless, consumes less power (my electric bill almost doubled with a second PC).
The other main reason (at least for me) for planing to buy a PPC for Linux is that 99% of security exploits is written for X86. With a PPC I'm saved from at least 99% of attacks, which are done by script kiddies, who just download some automated tools for X86 and don't know anything about hacking...
If the new Mac runs Linux (or FreeBSD), it would be a serious competitor, as a Pegasos is too expensive for me at the moment...