I think expandability has a lot to do with it. I've got a 2008 2.66 quad core Mac Pro
which has been the studio (and now office) workhorse for ages. I've got 4 drives in it and
it runs a printer, scanner, firewire camera and video converter plus various dongles. It
also runs my newly acquired copy of Logic happily but struggles far more with DP and
Protools when using VSTis. Replacing it recently in the studio with a Macbook Pro has
given me a lot more power but far more headaches in terms of attaching peripherals.