Yup, Tompa is right. These are collectable and it's not very hard.

Too bad I have no idea of getting the bananas att glimmer's galleon. But the bananas in Castle Crush that are inside the wall, are actually collectable. Through severe glitching of course.

There's also some bananas at Windy Well which isn't normally possible to be found but with glitching they can be found. Wait some time and I'll post some screenshots.
Edit: Oh wait, my memory failed me. There aren't no bananas inside the walls of Castle Crush. My bad.

This can be done by using a trick that allows one to jump multiply times in air. It as simply as do an upward throw and when the character lands you'll press exactly when she/he lands. It's very hard and requires perfect timing. 3 or 4 jumps is neccessary.
And I think I know why these bananas are there, I think they are originally placed there for being collected from the bonus - even though they are not collected when entering the bonus.
// Arne