For those just wanting the TGS demo, I can't see why. You have one track and very few cars to choose from. While I like R246, to most it's kind of a boring track. I completed the Forza 3 demo in a few hours, and now it's worthless to me, and this is what the TGS demo would most likely be like for you, a few hours of fun. There's a lot more to play with in Prologue, but if you've quit playing that, what makes you think the TGS demo would entertain you for more than a day?
But a good Spec IV update would get people involved in playing it again, if you merged the TGS demo with Prologue and gave it some of the online features of GT5. The Subaru damage would have to be taken out, since it only really applies to the Subaru, but I don't think it would take more than two weeks work to do this.
It may not happen, and I wouldn't cry if it didn't. I'll have FW3 to tide me over till GT5 shows up.