The main reason for doing this is the price of the processors. A 3GHz Quad Core Xenon E5450 runs around $40 on Ebay, while the comparable Q9650 Core 2 Quad runs around $90. So for about $40, I was able to upgrade my pokey 2.13Ghz system to a quad core 3Ghz system - and bumped up my windows "experience" score by 2 points. The only drawback I can see is that the Xenon is multiplier locked, but I could care less about that. The bus speed can still be played with to OC the chip, but I don't want to invest money in a cooler and the stock config will give my PC a couple more years at least. Huge thanks to whoever found this mod and also to the folks who run the site linked below which has a guide for doing the mod.
Note this mod does take an "adapter" which is basically a sticker, and requires some cutting on the socket since the Xenon has notches in the wrong places. My motherboard supported the new processor without resorting to any BIOS mods - which is probably something I was ready to try if I had to, but only because my PC has a dual BIOS :D The above link also has a list of compatible motherboards.