Margarine?! 
YUK!
This house is a margarine-free zone, but I do use BOTH butter (real butter) and mayo on sandwiches. Butter to 'seal' the bread, and mayo to moisten the fillings! 
Yes! That is my feeling as well, that if you're going to use it, use the real thing... butter. But I still think mayonnaise belongs on sandwiches with meat, cheese, and/or vegetables. I ask you, what is a good BLT without mayo? And mayo makes that leftover Thanksgiving turkey sandwich the treat it is.... mmmmmmm (IMHO that's the best part of Thanksgiving anyway - the leftover turkey for sandwiches).
Margarine, at all, in any way, is an abomination.
But, so is butter on sandwiches. I'm pretty sure it's actually poisonous in certain combinations-- especially with peanut butter & jelly. The butter actually becomes cyanide or something. I kept trying to explain that to my babysitter, the school lunch ladies, etc., but they never believed me. They just kept scolding me for wasting food!
REALLY!
Oh dear! I've been poisoning myself for years then, because a
really good peanut butter and jelly sandwich has butter on the bread under the jelly side...
... Great! Everything I like is either illegal, immoral, or going to kill you slowly and very, very painfully.... *sigh*
