Word-of-mouth is a valuable form of marketing, and the best way to achieve good word-of-mouth is to please the nerds/early adopters/hardcore fans. Avengers was a good example of this. While it is true that an unedited OT would not produce tremendous revenue standing alone, its use as a marketing tool and energizer for the hardcore Star Wars fans could pay HUGE dividends. This, among other reasons, is why I don't believe Disney would have allowed George Lucas to retain any rights with respect to marketing of the existing Star Wars properties.
As for Fox's involvement, I cannot think of any legal or business reason for withholding their permission, other than extracting as much money from Disney as possible in the process. In fact, it would be in their financial interest to prod Disney into a re-release before their rights to V & VI expire. Therefore, I am hopeful that the unedited OT will make an appearance prior to Episode VII, or at least in tandem with the VII-IX video releases.
I may not have been around for the theatrical release of Star Wars, but I watched the VHS copies of the OT dozens, if not hundreds of times as a child. I cannot watch the "special" editions, and those that think this is an overreaction do not understand what it is like to have an emotional connection to a piece of art. If Disney can convince me that they understand and respect that connection, I will be a vocal supporter of their Star Wars endeavor. If not, I will continue to assume that any future Star Wars movies will be typical, Hollywood cash-grabs, and not worth my time. Again, I realize that my presence/absence at the theater doesn't make any difference, but the collective marketing power of emotionally invested fans can make a big impact.