There is difference, but the difference isn't necessarily in the quality of the player. A seventh rounder is someone the team chooses to take a chance on. An UDFA is a player who might have offers from many teams. In many ways, a UDFA is in much better a position than a 7th rounder.
Dennard is a fine example; also Edelman. We used 7th's. Maybe we would have gotten these players if we didn't draft them, maybe not.
So, 7th's aren't greatly valuable, but they do have value. I would note that most of the compensatory picks are made just before and just after the 7th round, so these picks are certainly considered valuable by some.