Heck, I'm not even sure that I do.
Bombadil demonstrates the complete lack of power that the One Ring has over him by putting it on, and failing to become invisible. The ring has nothing to offer to Bombadil, therefore there's nothing there to be tempted by. And it would seem that because of this, Bombadil would be the absolute, best possible Ring-bearer, and it's suggested during one of the Councils that Bombadil should be recruited for the task. But he's ruled out by Gandalf (if I remember correctly) because it is felt that Bombadil has other priorities, and that he might not take the task as seriously as it needed to be taken (I'm working from memory here, so forgive me if I'm screwing this up).
So, is Bombadil there in the book to point out that we aren't or can't or shouldn't be entrusted with missions or tasks that we don't have a stake in?
What's your take on Bombadil?
I won't try to guess what the average movie goer could or could not understand, but I will say that I read the trilogy four times before I came to see Tom and Goldberry the way I have described them here. Whether I'm average or not, it definately stretched my brain cells.
Shalom.