Do you continue getting exited when you listen: “The Macarena” or “We are the World”? most likely, you skip those every time you listen to those in your iPod. (I would too)
You don’t delete them because when you sit in the computer you are not going to search among the million of songs and you for sure don’t remember now which ones they where.
You can create in iTunes an smart list that will aggregate songs that you skipped. Next time that you sync your iPod to iTunes they will be there. Then you can choose delete them or unmark them so they don’t sync (assuming you have so iTunes only sync check songs.)
Next time you skipped “The Macarena” you will finally be over to delete it for good…