I actually think the missing pieces are the least important and I don't think they add much to the play value at all. I also believe that the price is now lower which makes this a much more viable purchase. I can't say much about the current backdrop though as I haven't seen it in person. The older backdrop is very sturdy, but also very heavy and difficult for even me to set up even with my adult arms. I don't think it is something a child could easily carry around, which may be why they changed it. The new backdrop has more folds in it, which might make it easier to carry about and also easier to keep standing up. If you can ascertain if the new version of the backdrop is also sturdy, then I would go with that set.
As for whether you little ones would actually play with it, I think that depends on whether the child loves horses or not. I did and a set like this would have been one of my absolute favorite items when I was a child, but my dd has never cared for horses, so this would be terrible gift for her. HTH