Reasons to hack, or "root," your Android handset: Custom OS upgrades, PC tethering, full-phone SD backups. Reason not to: It's really scary. At least it was, until now.
It was only a matter of time before this happened, but the T-Mobile MyTouch 3G has been rooted, i.e. cracked wide open, meaning that it's ready for all kinds of customization that the stock Android installation keeps out of reach, including cooked ROMs.