Jared Spurgeon is a proven 24-25 minute NHL D-man that plays in all situations, and...is a significantly better player than Grzelcyk (particularly defensively). He is the exception to the rule, not the norm. He also makes nearly twice as much money as Moore and about four times as much as Grizz. Other than actual physical size, the two players have little in common.
Last season, the Bruin's PK was 3rd in the NHL at 83.7% utilizing larger D like Chara, Carlo, Miller, McQuaid and Holden. The smallest of these guys was Miller at 6'2".
The Wild PK was 13th in the NHL at 81.3% with guys like Spurgeon (among others) on the PK.
The B's were obviously doing something right on the PK. So, if they want to add another bigger guy into the mix (Moore is also 6'2") to replace Holden or possibly McQ's PK minutes, who am I to argue?
And as far as bang for the buck goes, here are some of the comparable contracts to Moore's from CapFriendly:
CapFriendly - NHL Salary Caps
Notice McQ at $2.75m and Miller at $2.5m, both of whom play less minutes than Moore, and signed their deals a few years ago. For what Moore gives you, $2.75m is very good value in this day and age.