The thing I take in consideration with Grupe is his exceptionally small frame. If he takes a hard hit tackling, getting tackled, or getting blocked, what are the chances he gets injured?
To me, the decision comes down to weighing the odds that Lutz misses more kicks than Grupe vs the odds that Grupe gets injured taking a hard hit at some point during the season.
He's a kicker. All he has to do is get in the way. If you give up 1 or 2 returns over the course of the decades he can play because he's not a safety, I think you live with it. The upside is worth it.