i've experienced this, and actually, although it annoyed me, it made the job of whittling them down much easier...especially if they are really bad.
it gives you an insight into the true character - rather than the performance they put on in an interview or the CV and letter they got someone else to write.
i don't mean a few minor errors, i mean terrible ones - textspeak, no greeting, apparent rudeness, arrogance, stupidity, lies etc etc
many times i've seen someone with a great letter and CV and they've been a terrible worker and its become evident they didn't write either of them themselves - they couldn't have!
i did think of writing to correct a few people too, to help them, but actually its useful.
let them learn the hard way and figure it out for themselves...let them learn the art of making an effort and using common sense - it shouldn't take them too long if they have any sense.