I always make sure to have 60 guys on my preseason roster. If someone gets injured, then you have a capable backup to start the regular season instead of what's left of free agency. And, as beercloud said, they can be used when you pull the starters in your preseason games.
The only exceptions to this, for me, have been on a few occasions when I have really needed to cut guys during the preseason due to salary cap issues.