I do it. I think of it as just a sort of a memory thing to help me recall who left and whatnot. Still, you don't get them that often, and that's one of the reasons why I keep them.
Yes I keep all letters I deem important, including move out letters, birthday letters, and just ones that make me smile I used to keep EVERY letter my villagers sent me until I ran out of space, then I had to weed out the boring/useless ones (there were many, lol)
I don't keep them, but the one I got from Francine when she left I had to. She was saying how it was too painful to say goodbye to me in person or something and it made me sad.
I save them, and other ones. I even go back and read them.
My favorite is one Daisy sent me about how 'there's not a kind person in the world that doesn't love plants' with a houseplant attached. Feelings were felt. ; ;
I keep all the leaving ones, even from the villagers I only had for a little bit of time because of cycling, and occasionally a letter that made me laugh or smile or whatever.
I save all of my letters, no matter what. Even if it's a Villager I dislike, I still keep it. The only ones I throw away are Bug/Fishing certificates, Crazy Redd/T and T, and HH Showcase.
I keep most of my letters. I definitely couldn't get rid of their moving letters, though. I even like keeping my moving letters on my person at all times.
i save the letters that my villagers spontaneously write me, the ones that make me smile, and the move out ones. the rest go in the trash since they're all like, oh here's some furniture.