I think it is a tradition for them to release one of their games for free every 10 years or so. Gotta wait a few years...
Yeah, they started it with Arena on the TES 10th anniversary, and then with Daggerfall on the 15th anniversary. If they do continue with it, then the earliest we will see Battlespire is on the 20th anniversary (2014), if we are lucky.
Actually, I'm pinning my hopes on a Steam release on 11/11/11 or before. Because of http://bethblog.com/index.php/2011/02/18/ladies-and-gentlemen-start-your-creation-engines/, specifically this part:
If anyone is seriously considering this, we’ve got a few ideas in mind for a collection of quest rewards. How’s this for a shower gift: a Steam key that will grant you, and presumably Dovahkiin him/herself, every ZeniMax/Bethesda game — past, present and future — for life.
If they didn't release Redguard, Battlespire and the Travels, I would complain
I already have that Steam key. It "only" give access to the games published by Zenimax (including Bethesda & iD) that already are there.
In any case, I very much doubt they will release Battlespire on Steam, Arena and Daggerfall weren't released there either. Infact, none of the Bethesda DOS games have been released there.