What a great question! And one that's been lurking around in my brain too! Kinda existentialist (sp?) answer but I like them because they ARE! I can't really explain but we used to see them when we were at WDW with the art deco style and imagine being on them going off to a great sea adventure. When we did our first land/sea cruise, I can't describe how we felt to be boarding that wonderful bus at the Poly - we were the chosen!
On our third cruise in Aug we're driving to port and I know that we'll have a fabulous time but I'm already missing the feeling of being on that bus and knowing that others are looking at it and wishing they were in our place! (OK, I'm a selfish egomaniac - I confess! BTW I want a copy of the video they show on the bus!)