There's nothing wrong with penciling Bennett onto the 3rd line next season, as long as the coach doesn't get married to a lineup when players start exceeding expectations. Bennett (as much as I like him) hasn't yet shown he can produce as a top 6 player. So instead of giving it to him, let him earn it. Worse case he does, and there's no room in the top 6... then the GM can either make a move to trade someone for additional assets to make room, or we finally have some depth down the lineup.
The issue with Despres, is he was penciled in a depth role, but due to Maatta taking his spot and Bort being waiver eligible, there was no room for Despres. Had Maatta gone to juniors after his 9 game stint, we would have almost certainly seen Despres up all year. But with the signing of Scuderi there was just no room. Which was stupid. Play the rookies all season long, then look for a depth vet guy(s) at the deadline if there's any doubt in the rookies (such as Mtl with Weaver). Crosby, Malkin and whatnot are enough to get us into the playoffs.