The collectors editions came with some exclusive content which now also comes with the single player DLC. Also some promotional stuff like an artbook.
I would recommend those games, yes. I don't think they are as good as ACII, personally, but it is all part of the same story and all continues Ezio's life, and Revelations also wraps up Altair's story. In addition, Desmond's story continues to progress.
So yes, they are important to the plot and you should play them all.
^ Cut Lost Legacy out of that list. It was canceled and its story was used for Revelations.
edit: I wouldn't be too sure about the development of a Vita game just yet either - we've heard nothing about since it was announced. Also, we don't know which AC title (if any will, for sure) will be made for Wii U.