Reasons to buy a modchip:
1. If you are a developer, it is nice to switch between retail / debug bioses. This can be done with PBL of course or with a switch connected to the tsop, but it is more convenient and more configurable with modchips.
2. Easy to turn off if you want to play live games. The X2 bios prevents you from screwing up and accidentally connecting to live, but it doesn't allow you to play.
3. No chance of breaking your xbox due to a bad flash. Yes I know everyone says "I did X xboxes with no problems" but it only takes one time to be a massive pain in the arse. Every time you upgrade your bios you take this risk. With a modchip, the worst that can happen is you screw up your modchip's flash, but with the Xenium that doesn't even matter.
4. Lots of potential other cool features, like the spi on the xenium.