Beagle is better than they are. You can argue Benning shouldn't have paid 12m for him, but saying other players are "serviceable," "filling in just fine," etc is meaningless.
It is only meaningless to those that don't believe in managing with future cap issues in mind.
I think getting 4th line players that are just fine or serviceable is an important part of building a team. Of course, you get the best in those roles that you can subject to leaving yourself money to pay the more important players.
Notice the Stanley Cup champions this past season paid their 4th line center $1.75 million for the year and didn't re-sign him, so he left for the money and term that some desperate general manager would give him in Free Agent Frenzy. This season the Capitals are paying their 4th line center $650K. The Leafs are paying theirs $925K.
The Canucks have the cap space to pay Beagle this season. This season isn't a problem and the Canucks are probably a better team with Beagle in that spot than most available 4th line alternatives-maybe even all available alternatives. Somewhere down the road if their players are any good they'll have to pay them. Ignoring the cap in the future because it doesn't hurt in the present is a fool's way to manage a team, as is building your future team by obligating yourself to pay an aging 4th liner way more than 4th line remuneration for four seasons.