As others have explained, for several reasons, it's the uber weapon. It can do pretty much anything you want it to do- finesse, trip, disarm, attack enemies far away, attack enemies close up, two-hand power attack, everything. Heck, it's an awesome weapon even if you're not proficient, mainly due to the ability to AoO trip anyone who comes at you (trip only requires a touch attack, remember, and who minds taking a -4 on a touch attack?).
And if you're a high-strength Fighter with Improved Trip and Improved Disarm, well, you win.
You just win. Simple as that.