It was designed by Salazar to help his heirs cooperate - Presumably, all the obvious ways to fuck with it are well and truly walled off, because he'd expect a bunch of them to be morons about this and try to light the awesome gift horse of a working trust engine on fire. Also presumably, it isn't an universal oracle, the world not being currently ruled by Salazar Slytherin the God-King. At a guess, it either keeps a log of your life separate from your memory, or the ability to speak parseltounge depends on an intact mind, and the only thing you are able to say in parseltounge if you do memory charms on yourself is "I'm a moron who thinks obliviate is a toy".