Promo Pokemon are fun!!! Collectors love them. Also, as someone already mention, there is no way someone can catch a Tropius in D/P. I smell a Tangela w/Ancient Power give away in the near future.
If it's in a "weird red ball', then that's probably a Precious Ball which to my knowledge isn't found anywhere in the game.
WildFang, there's some people from Japan here.
Darn I was going to say the Chatot thing. I can't remember when, but a while back they gave away free Chatot's when you can easily catch them in I think Route 204.
While nothing too special may be given away they are a time when people can get together and have fun. For example the chatot given away at the concert seems to be more of a souvenir so that the people who went have somthing they got for free to remember the concert. It may not be special in the sense that it's unusual but it is special in that it holds good memories.