One very smart reason to release it... (Score 2, Interesting)

is to shut up all the 'if only Van Buren had come out!' talk from the Fallout faithful, both now and after the Bethesda game ships. The fact that it's obviously a flawed, unfinished POS engine and toolset combo should lay some of the idealization of the 'potential' Fallout 3 to rest. And of course it gives the truly crazy Fallout fans, who would never buy the Bethesda version but would complain every inch of the way, something to chew on for the new few years. Brilliant!