It seems that there is a general trend towards sandwiches being a free-for-all when it comes to ingredients. It's too easy to just mash things in between two pieces of bread and forget about it. "Oh, there's a piece of shattered glass in this sandwich? Great! Can barely taste it!" It's the ultimate way to hide things. Got stale bread? Toast it! Now it's stale on purpose.
Sometimes when you're making a toastie (grilled cheese), the cheese and bread just isn't enough. I usually opt to add sausage or chicken. I guess if all you have it banana, that's just as good too. The cheese is already going to be squishy, so I guess the banana will just blend in. Still, I think I will just add extra cheese the next time instead of adding the banana.