From a developer- or publisher-POV adding a multiplayer component or at least a powerful modding device is the sensible thing to do. Because it can turn an otherwise reasonably successful game into a huge hit, establish a large fan community, and so on. Of course, if the game is a failure by itself, multiplayer or modding tools likely won't change anything...

Personally, I'm perfectly content with playing a good solo player campaign. Yeah, I know, I'm a minority.

Only very seldom I find myself tempted by mods or multiplayer. Or even developer-made add-ons; only when they really add to the original game, story-wise that is. Shivering Isles for TES4: Oblivion was one of that rare occasions for me.