After a 4-for-5 performance yesterday, D'Arnaud is currently hitting .297/.364/.453. For a decent defender at shortstop, that's really good. D'Arnaud is 24 now - if he can improve the major-league team, that's probably where he should be. Since he's entering his mid-20s, I don't see much reason to worry about whether such a move would affect his development.
With Pedro Alvarez and Steve Pearce out, there's an obvious hole at third base. (Brandon Wood has actually hit very well in a small sample this month, but his overall OPS is still .552; as everyone knows, I'm not a believer at all.) I'm not sure, however, whether D'Arnaud can play there, since he hasn't played much there in the minors. He has a pretty good arm, though. He also played third at Pepperdine, and thought the transition from the middle infield to third wasn't terribly hard.
There's also the possibility that the Pirates could simply have him play shortstop, and move Ronny Cedeno into a bench role. Cedeno still has just a .616 OPS overall, but his defense has been solid this year (last night's error notwithstanding).
Another possibility would be to call up D'Arnaud and move him into a sort of rotation at shortstop and third, starting about two-thirds of the games. This would probably be the option you would pick if you liked the idea of promoting D'Arnaud but thought that Wood or Josh Harrison were worthy of regular playing time. (And for what it's worth, I'd still like to see more from Harrison, who also hit really well at Indianapolis but has only had a handful of games' worth of at-bats.)
In the end, I might be inclined to bring D'Arnaud up and have him take a few starts from Cedeno at short, while also taking some starts from Harrison at third. You can argue for a number of possible roles for D'Arnaud, but in any case, there's enough of a confluence of ability (D'Arnaud's performance this year suggests he can function at the major-league level) and opportunity here that I think a promotion is quickly becoming a good idea.