People like to know. They like rules: when this occurs, do this. X implies Y. Moreover, we like to believe that our knowledge, including our knowledge of convention, corresponds to reality. In this regard, I'm not sure I see a difference between prescriptivists and descriptivists.
On the one hand, I see the advantage of just telling us how words are used. On the other hand, I can see that in some cases, we need to know if we are not communicating effectively (and there are many ways this can happen, some of them less obvious than others). A possible example is the contention by some people that there are sexist elements in our language and we ought to try to eliminate some of them. I could do without the lectures and some of the remedy grates, but generally I don't have a problem with their bringing it to my attention. I have my own pet peeve: the interminable conflation of 'schooling' and 'education' which are used synonymously. I have a dilemma here: the very fact that someone does not see the difference or believes it to be negligible is a huge impediment to solving problems related to actual education. OTOH, people don't have patience to learn a different way of communicating, particularly when you're disagreeing with something they already know to be true.
The upshot is that when I point out this source of confusion and poor communication I am often being confusing and communicating poorly. One potential criteria to use for determining when to correct is: does the usage create more confusion or does the correction?
