I see two main reasons for a PS3 in particular when you've got a gaming PC.
There's the Blu-Ray player, for one... It's not just any player, it happens to also have some expensive hardware to decode and filter it, making it one of the higher-end DVD players... Which is good, because while you can easily find tons of Blu-Ray players for under the $400US price of a PS3, (I've seen some under half that) you can't find any other GOOD ones until notably above that price point.
The other, of course, are the tiny handful of PS3-exclusive games. There are pretty much no Xbox 360 exclusives left, particularly those that would come to PC, since one must remember that while Microsoft has a vested interest in their console, they also have a much longer-running interest in the very operating system they make. Of course, these games are not terribly numerous... To me, the only one that stands out is Metal Gear Solid 4. Of course, with the virtual guarantee that it'll come to the Xbox 360 in a year*, means that if you've got an Xbox 360, you won't need a PS3 for that game unless you're really itching to play it ASAP. 'Course, I have neither system, so I'll likely have to "borrow" one from someone.
*Yes, I've seen plenty of PS3 fanboys argue against this, but I view it as inevitable; the PS3 will *NEVER* have the 140+ million-unit sales level of success that the PS2 had, or even stand a chance of coming remotely close. It's safe to say that unlike the PS2 and Xbox, the PS3 will NOT be out-selling the Xbox 360 5.8-to-1. As it stands, Sony is still BEHIND overall, and at best could really only get as far as perhaps 1.25-to-1. Sony's bold claims of selling 150 million PS3s, when they have about 1/10th that after almost 2 years in, (and no production shortages) Would imply that they'd either have to go on for the better part of 20 years like this, or Microsoft AND Nintendo would have to spontaneously cease operation. So every day that either/both of them continue to run makes such odds ever slimmer, until in not too long it'd be impossible because the 7th generation console market will become fully saturated and dead before they'll have a chance to sell that many.