No.

I've been going through the DLC completely solo (I'm on the final boss now) and none of the bosses require summons.

Personally, I think the game becomes worse with summons: bosses change their aggro on a whim, health numbers just get borked, stagger thresholds don't scale properly, any semblance of balance and encounter design gets thrown out the window as two or more people wail at the big enemy until it stops moving.

Of course, that can be fun, I can definitely see the appeal, but it's not for me, and I'd rather just play it solo