I'll go along with mustard. I love it, too. And it goes well with cold, deli style sandwiches, especially with paper thin sliced beef or ham, and cheese...even bologna!
But a Reuben is a hot, half meal prepared as a grilled sandwich. No pickles or lettuce or tomato.
In high school, I had a girlfriend that put mustard on her french fries because nobody would swipe a few when we would get together at Max's drive-in after school.
She didn't like it either, but nobody snatched her fries!