Well, just to clear some things up, there actually are a few reasons why someone would get an XBOX 360 for the sole purpose playing FFXI (although I assume most people who will be playing the 360 version will already have the console in the first place):
-Sony no longer makes the "old" Playstation 2 consoles. Instead, they only make the new slim versions, which do not have a slot for the HDD (so if your PS2 or your PS2 hard drive dies, Ebay is just about your only option).
-It takes a fairly fast computer to play the game at its highest settings--probably a setup that would cost about $500. This would make the XBOX 360 version the cheaper choice, strangely enough.
-The 360 version will have a draw distance twice that of the computer version's maximum. Of course, you can download a program on the PC that allows you to set the draw distance much further than even the 360 version, but not everyone does that.
-It would be really nice to play the game on a high definition TV while sitting on a couch instead of a computer chair.