You're correct. There typically isn't a lot of direct derived from a 7th round pick. The Pats have picked up:
Patrick Pass, FB, 2000
David Givens, 2002
Tully Banta-Cain, 2003
Matt Cassell, 2005
Brandon Deaderick, 2010
Alfonzo Dennard, 2012
and that's about it.
However, they can be "deal sweeteners" to throw at teams that want to trade up into a Pats' slot. For instance, a team that wants the Pats #31 might have to give up four picks to make the point value close to even, so the Pats might give them back a 7th to get it done.
But 7th-rounders are mostly useful for pre-emptively securing the rights to a prospect who otherwise might be a UDFA and, thus, avoid have to bid against other teams for him.
It actually quite often works out that the signing bonus money that a team needs to give to a UDFA to persuade him to sign with the Pats rather than with some other team, raises the UDFA's compensation above that of a 7th-round pick. It might only be $100k or so, but that $100k comes out of the team's CBA-limited rookie-pool allotment and, thus, can't be used to "bribe" another UDFA.