You're talking a lot about this...reason for the pause system. I'm inclined to agree with the other user, earlier, who stated it's a vestige of the turn-based system, rather than associated with controlling multiple characters. The existence of the Tactics system might imply the same--it's a way of automating all non-player-controlled characters, thus alleviates the "concerns" of controlling a party, thus the pause isn't for that. It's an academic rebuttal, I'm not sure we'll ever know which it was.
I always felt the vestige for the turn-based system was the extra opt-in option to literally make the game pause when a character's combat round is up. It's also an accessibility thing, though I'm not sure how much that factored in in 1998. I tried it, but I quickly found that irritating though, as I only preferred to pause when I felt I needed to make a decision that needed puasing. I didn't care for the autoattacks to all be paused, which was a very regular occurrence once the party got up to 6. Due to the nature of the round system, however, for a single character I often had a 6 second queue to worry about my next attack. So with a solo character (or even just two), I didn't need to pause as much as when I had 6 and needed context switching for the character's abilities and so forth.
In a multiplayer environment, though, I don't need to pause to evaluate if there is aggro on other characters because those players are making those evaluations on their own. And if they aren't making them as good as I will, maybe something bad will happen and we'll all have a laugh and we can try again. (My tolerance for failure shoots up in co-op. Heck, my tolerance for a lot of things goes up if I'm playing with friends. I have literally played mediocre games that I would never play alone simply because they had co-op. Co-op is a giant force multiplier for fun for me).
Co-op got me into Dark Souls. Co-op got me into TOR and WOW. It got me into Dawn of War II. It got me into System Shock 2 (and from there, the Deus Ex games!) If anyone wants to play some Civ5 or EU4 MP, I'll be all over that. A game like EU4 or CK2 I actually roleplay the nation I play. I love those games, but sharing the experience with a friend (or many friends) is priceless.
I actually don't co-op game a whole lot, specifically. But in a world where I find there's a lot of games out there, co-op is a compelling argument for "trying something I'm not sure about."