My favorite part about mappy that I didn't see on FFXIDB was that you could see what other players were targeting.
Not that I would ever do it myself, but hypothetically if one was fleeing around at 300% while invisible, that horrible person would be able to see if another player was targeting him, and know that he needed to lay off his evil hacks for the next few hours.
If you were camping an NM, you could tell whether another player was just passing by, or trying to pop/camp, as mapppy would show their target line rapidly shifting from target to target as the player hit tab.
I didn't realize it was still being maintained or that it worked, though.