I don't use butter on sandwiches. I will use a little mayo on the various sandwich salads, but just enough to hold it all together.....like chicken salad, egg salad, tuna salad, and ham salad. I like seafood salad only if it is not too mushy.
I will use some mayo on the bread for cucumber sandwiches, tomato sandwiches, and sandwiches made with fresh baked chicken or turkey that's been sliced right off the bird. Otherwise, no mayo for me on any deli meat sandwiches. And definitely no mayo on a sandwich containing cheese. I like my sandwiches generally without condiments, or sometimes with a nice mustard. Gotta have mustard and ketchup on my burgers....no mayo.
My brother will eat absolutely no mayo on anything.