I don't see any reason to leave
any pencil markings on the music after one is finished with it (except for the tagging thing that Wilmer mentioned, that's cool as long as it's nowhere near the dots). I really don't care what the previous guy thought needed to be highlighted. I'm a good enough musician to make my own markings, and I usually find there aren't that many needed (the important stuff is almost always printed on the page anyway). Why can't people just rub their notes out after the concert? It really doesn't take long.
It makes you laugh sometimes, it makes you cry sometimes. A few months ago I played Ravel's Bolero with a local orchestra. The last guy had written in
all the notes transposed for a Bb trumpet (the parts are for trumpet in C). That wouldn't have bothered me, but he got it wrong! The notes were all written in a semi-tone up.
I couldn't believe it! What's worse is that I didn't have time to rub them out before I played (in the first rehearsal) and it was very distracting. Funny though, the guy on first actually thought that they were correct and started playing them.
Sorry to rant, but a lot of these pencil markings actually make it harder to read the music. Which is a major problem when sightreading is so important in our game.