It's sort of a weird observation, because having an excellent defensive skill is part of being a good defender. And if Gurriel's arm prevents players from even trying to take an extra base, then the other team is taking less bases. This is good.
Also I think Gurriel could still become a solid all around MLB OF. They had enormous patience with Teoscar and it seems to have paid off. Gurriel played so much IF in the past, he still has something to learn out there. I'm also not sure why they've never given him a shot in CF. He's fast, has a good arm, and used to play SS.