When I was a kid my mother always made my sandwiches the same way; two slices of Wonder bread, a slab of meat, and a slice of cheese. Forget everything else.
As I have grown older, my tastes have changed, and I make my own sandwiches a little differently. I now use whole grain bread and I actually add romaine lettuce or alfalfa sprouts. Sometimes I add a slice of tomato or avocado and I top it off with some designer mustard (never did like mayo).
Because of tradition, I still always add a slice of cheese. Cheese is one of my favorite foods and I would never think of not adding it to my sandwich!
One day I didn't have any cheese (a national disaster) and made my sandwich sans cheese. After I finished eating I realized that my sandwich didn't taste all that differently than it usually did with the cheese. I guess the way my sandwiches have evolved over the years with all the yummy additions has really masked the taste of the cheese.