[TomGullen]

IANAL but if Blizzard is aware of this copyright infringement and do not act on it, the developer can go for an acquiescence defence. If successful, Blizzard surrender their rights in that particular project.

This could potentially have an effect on the value of their IP which can give leverage to potential acquirers/investors.

[extra88]

No. There is no such defense in U.S. copyright cases. Again, only trademark requires them to pursue infringement to maintain their protection, and by extension, value.