Sometimes, but very very rarely. Mostly just to decorate their homes or pickup extra items from the floor and whatnot
Right now they have their pockets and dressers full of hybrids I've been too lazy to actually move around and plant
Sometimes. Not often though. Usually just to decorate. They only thing that happens if you don't use them is rarely villagers will ask where they've been.
I've recently made a mule character for paths. I only intend to use it to talk to Sable each day to get the QR sewing machine unlocked for my paths, and it will also be used as storage. Otherwise I haven't talked to any of my villagers with it, and I don't intend to either.
I log into one alt almost daily - I use the alt to get further items off Sahara, Redd (if he has more than one real) & a fortune of Katrina (as far as I know your alts fortunes are included in the 20 visits you require for her to set up in your town).
I tend to order from Timmy/Tommy Nooks & HHA via an alt - to keep my mayor free to get letters off villagers/Gulliver/Redd items.
My mayor sends one letter daily to each villager, so her letter box can be full on a daily basis. My alts don't send letters.
Together with storage space/pattern holding make them indispensable.
I created one mule character last year so I could have more paths, but ended up deciding that I'd play with her too. I don't always use her ( sometimes I stay 4-5 days without using her) but I always do when I am home and have nothing to do I'm making her house larger rn, she has tons of gracie clothes + is friends with all villagers.