Are multi-author signings worth the time and trouble? It depends what you expect of them.
For me, it's not really a matter of selling lots of books and making lots of lolly. I'm making my name and my books known to people who otherwise might never see them. Maybe they'll buy one book, and maybe they'll go on to buy more elsewhere. Maybe they'll pick up a bookmark or a card to give to a friend who likes "that kind of stuff." So future sales are one aim.
If the signing is sponsored by a bookstore, you're making sure they know of your existence and hoping they will carry your books in future. If they already do, you're reminding them how charming and cooperative you are. With luck, they'll be not just stocking but recommending your books to their customers in future.
Even if there's no bookstore involved and you sell hardly any books, you're meeting fellow authors, exchanging ideas and tips, making new friends--not always easy to do when you spend most of your waking hours with the imaginary people in your head, trying to turn them into words on a screen!
Sometimes a multi-author signing--or any signing, come to that--is at least partly an excuse for a couple of days away from that screen, even if you never escape the people in your head. Last weekend, a 50-author event at a bookstore on the Oregon coast gave me an excuse to drive over, spend the night, and take a couple of lovely walks on the beach with the dog. Good enough reason to accept the invitation, though I did sell some books, too!
Yes, Mum, I've been rolling in the sand
Gonna shake that sand right outa my hair
Encounter with a hippo
Encounter with a ?jellyfish?