He has below average career numbers because he was the starter. We aren't expecting him to be a start nor are we paying him too be one.
Allen's contract this season is bad, St. Louis was in a bind, by trying to get ahead of the Market, Bergevin paid a price that he didn't have to. He could easily have forced St. Louis into paying an asset to move Allen, and they could have gotten just as capable a back up with nothing more than cap space.
The issue isn't Allen on his own though. It's his rapid approach this offseason. Montreal had tonnes of cap space, several players on expiring contracts, tonnes of picks. They could easily have waited to see how this unknow flat cap market shaped up, and done really well for themselves. Toews was just moved for two 2nds, Schmidt for a 3rd, several teams have good RFAs that could be offersheeted, or pressured into trading, and there are still decent UFAs on the market. Montreal could retained salary on some expiring assets, making for very good assets in the trade market, and filled holes with value signings, and well below market trades.
Instead, they blew their cap space on paying an asset for Jake Allen, using a big trade chip to overpay Josh Anderson on a risky long term deal, and traded a pick to get the signing rights to a defensemen who is a 3rd pair guy at best, all so they could massively overpay him on a 4 year deal. This was a massive opportunity, and Bergevin's biggest coup was getting Toffoli.
Bergevin must feel like this team's window is the next 2-3 years, and maybe he's been given a short leash so it's going for broke. Either way, this team's cap is very poorly managed. If the young players are good enough to actually make them contenders in the next 2-3 years, having Edmundson, Petry, Anderson, Price, and Weber on their current deals when you have to pay them is going to force them to take a step back.
This team is probably better next year than it was last year, but that might still only make them a bubble team, and they are betting that they are much better than that, I just don't see it.