Never used to, and then i got a (very) mild scratch on my oleophobic coating so I put a Spigen SGP ultra clear protector on.
Coming from the old Zagg and GhostArmor days, I was anti-screen protector until I started using the SGP crystal on my phone and was impressed with the clarity.
It was relatively easy to apply (dry application) and you can't tell that there's a screen protector on. I like it for the little bit of added protection. I would say resale value, but hey, I'm holding onto my iPad 3 for another generation or two now.
I never scratched my iPad2 in the year I had it - no screen protector on, and I used the Smart Cover for both. I probably wouldn't have put one on my iPad 3 but the tiny scratch bugged me.