Poolman endured the "trial by fire" and is only recently (and for a bit last season, to be fair) looking like he belongs, albeit perhaps not as a top 4 D. If you want your prospects to develop you've got to trust them at some point to play in the NHL, despite concerns about some aspect(s) of their game.
I'm continually surprised by the circular arguments I see here regarding prospects: "X isn't playing in the NHL because X hasn't proven that he can play in the NHL because X hasn't played in the NHL." WTH?
Play him. Give him some games without the pressure of benching if he makes a mistake or two, with the knowledge that even our best D have made their fair share of howlers. Coach him after every shift and after every game. Go over areas of weakness and strength and watch for adjustment/ improvement. Even the worst D isn't going to cost you game after game (as we know from this year so far). Then you know.