My argument is that people felt the whole "Brothers to the End" and Cole Train because of the campaign. Playing Gears for just MP would have had less of what it got IMO. It wouldn't have made any real sense.
Most MP is a continuation of the game or the weapons you acquired while playing the game. Sometimes it just doesn't stick with the game based on its aesthetics. Look at Resistance or even KillZone. People see campaigns and not multiplayer. Not everyone understands Quake or Unreal. I played Unreal 2's SP, but did everyone else play that or just stick to UT2k/2k4?
It's like you can fit in MP or you can just streamline it. The better MP is real because it's what makes people stay. Quake or even Halo are big contenders to that. Gears as well. You sorta need that whole package because you're pleasing one group of people and not the other.
I think there is a separation, like there always has been. I feel like Gears of War was a lot of what Halo or even CoD were before people just went their own way.
Halo owes a lot to having good MP, but that wasn't "Finishing the fight". It was a solid plot with this large scale while Gears of War was "Brothers to the End".
Apparently MP is up there with the Joneses. It looks better than it ever did, graphically, but it's still incredibly selective (still).
Was it worth it, that's the biggest question. I think any IP that had a solid run owes it to their SP, whatever the cost was. I don't think Bungie ever regretted their Halo campaign costs.