I'd say having both actually makes most games worse. I still see value in games that offer both, but things like numbered Battlefield games do not need a single player.
If you are tacking it on just to have it, don't.
I totally agree, generally speaking. The Battlefield 4 campaign is a joke. They put a little more effort into the Call of Duty campaigns (Treyarch in particular), but they're 90 percent garbage. I'm not sure if it was Call of Duty that started it, but at some point I remember reading that the main reason games like that even have campaigns is to generate footage the company can use for their marketing campaign.
I actually think it used to work in reverse to a certain degree (at least on consoles). People preferred lengthy single player campaigns before online was really a thing, and they'd slap on a multiplayer mode - which took far less work than the reverse effect and, I think, lots of times probably turned out pretty well. After all, as long as the core shooting engine is fun to play, it doesn't take a LOT to make a somewhat decent competitive experience. Or at least a cheap little diversion.
So now that I think about it I'm not a fan of slapped on single player modes, but I think I'm ok with slapped on multiplayer modes.