I disagree, OP.
Co-op is super fun. There is a really satisfying feeling you get when you and one or more allies team up and take down an annoying boss. I don't really use summons that much, but I have in the past. I don't think I used summons at all in Dark Souls 1 or 3, and I remember using summons for the Orphan of Kos in the Bloodborne DLC (although they ended up dying in the first 30 seconds of the match, so I had to face him myself anyway).
I agree that the boss battles are a highlight of Souls games, but they can be enjoyed equally solo or with a companion. The games are just damn fun to play. I wish there was a more traditional co-op system, because it's just fun running through the levels with a friend and kicking monster's asses together.
I usually do my first playthrough solo, then any new game+/subsequent playthroughs, I help out other players or co-op with friends.