I think from a business standpoint, there are arguments both for and against the demo. On the one hand people are going to know what they're signing up for this time, given these will likely be more or less the same as SM in terms of content etc, so you don't really need a demo.
However, that is precisely why we need another demo version: so they have a chance to show the differences/draw people in with story teasers, and entice more people to buy the game. You can do that with trailers admittedly, but demos let you sample the game firsthand and are a much more effective marketing tool.
I don't think datamining is an issue on GF's radar...if it was, they would have fixed it with SM's demo. That's more of an issue amongst fans who don't want to be spoiled than it is one for GF...any publicity is good publicity.