Here's why -- right now, I pull all my mail down on all three of my active boxes, the Linux server and my laptops. I'm planning on going Mac-only (well, more or less) for mail. Thunderbird on the PC mostly pulls down things I need for the side programming work, I don't care if it has everything, as I've taken to using the Mac mail client (almost) exclusively for actually composing mail. Right now, news is set up on the Dell under Thunderbird, but I don't like the interface all that much (not that it's terrible, really), and I've got mail and news globbed together on my Linux box with Gnus. I suppose I could just turn off the mail portion (those files are simply going to be dumped onto the Mac, they're all flat files so they'll still be easy enough to find) and continue using the Linux server for news, which in some ways is convenient (it's the only box that's connected to the internet when I'm at work, so that's when I use it), but eventually the whole box will be retired if I, you know, move anywhere interesting.
Speaking of which, I'm considering buying the new version of LightWave 3D -- this time, for the Mac. The price is damned reasonable (under $800), and I can afford it, and that will allow me to permanently pack up my Windows desktops, which at this point have no other use whatsoever (and only one of them at that). Then, I can use the (physical) deskspace to hook the 15" PowerBook up to the 21" monitor for a reasonable amount of (virtual) desktop space, and sometime down the road, replace that with a cinema display after I, you know, move anywhere interesting and dump all of those big heavy CRT monitors.
Mind you, a G4 isn't the best chip for that sort of thing, but it should be adequate (it's faster than what I'm using now), and anyway, someday if I had good reason, I could upgrade onto a bigger box. Of course, I don't know how solid LightWave is on the Mac -- but on the other hand, it was always kinda touchy on the PC side, too, so I don't know that it could be worse on that count.