What need? The offense has overall been fine. How is bringing up Benintendi going to help fix the pitching staff, which is the real issue?
Are you prepared to live with the kid struggling after he comes up? In the midst of a pennant race? In Boston? Because odds are that's what's going to happen. Bogaerts did okay for the 20 or so games he played in 2013, but struggled mightily in 2014. He started figuring it out near the end of that season. JBJ (who's career path was very similar to Benintendi's to date) issues are well documented. Betts did fine, but he also came up to a last place club; no pressure. Benintendi projects to be a good player, but he is not Trout or Harper.
Frankly, I think the professionals who see the kid every day are in the best position re. deciding when he is ready.