One of my GSTLs does this, and it drives me nuts! I realize it's easier than using the number pad (like texting on a phone's number pad without T9), but it can't be good for the screen. And the ink spots set off my OCD.
I was nice and invested in the DS styluses for the team to use on the LPDAs. We had 8 packs on clearance for 88 cents and so I bought them all.
Because I'm nice. And because I was sick of pen marks on them. But, they work well.