Grammarians fall into two camps: prescriptivists, who say that there are grammar rules to tell people how to use language; and descriptivists, who take the point of view that when they say, for example, “The verb ‘to lay’ takes a direct object, as in ‘to lay an egg,’ while ‘to lie’ means to recline, as in ‘to lie down,’” they aren’t dictating usage, but merely stating how a reasonably well-educated person speaks. If usage changes, the “rule” also changes, or it disappears.
I lean toward descriptivism in grammar—a lot of the “rules” were imposed by grammarians to make English look more like Latin, and they make no sense with a Germanic language. But I’m more of a prescriptivist when it comes to vocabulary.
Every time one word is confused with another, we lose something. There’s a difference between “uninterested” (couldn’t care less) and “disinterested” (has no stake in an outcome). If people use “disinterested” when they mean “uninterested,” how can we express “A scientist should be disinterested in the result of her experiment”? Someone who doesn’t know the difference between the two words might think that this means that the scientist shouldn't care about what she’s working on. She might care deeply, but she should remain neutral. You’re not advocating boredom in the lab.