Blizzard's rights in copyright will not be affected. Their rights in trademark might be, though.
Copyright law does not have any "use it or lose it" rules, or rules which require enforcement in order to acquire or maintain rights. Instead, it is an exclusive right vested in the creator (in this case, Blizzard / Activision / whoever).
Trademark, on the other hand, is probably what the people making this argument are thinking of.
Because trademark is fundamentally designed as a means of consumer protection by ensuring that consumers can rely on representations about the source of goods and services, it is very important that goods and services bearing a mark (e.g., a computer labeled "Apple") actually be from Apple, Inc.
Trademark is focused on "use in commerce": the offering of goods and services for sale. A "free to use" license for trademark rights is therefore a non-sequitor. Trademarks must be used in commerce, i.e., in exchange for money.
Also, trademark owners can lose rights by granting licenses and not controlling them carefully to ensure that the same or better standards of quality are met as with the trademark owner's offerings.
So, for example, McDonald's licenses the arches, design, and other names and logos to franchisees. But the Franchisor also closely controls the franchisees to ensure the product is consistent, so that the trademark is associated with roughly the same goods and services no matter where it is used.
Blizzard's trademarks would include WORLD OF WARCRAFT, BLIZZARD, probably every name of every expansion of WoW, etc., all of which are still extremely valuable.
If a third party offers a crappy knockoff product under Blizzard's name, this would be trademark infringement. If Blizzard does nothing about it, this can reduce the ability of the terms (BLIZZARD, WORLD OF WARCRAFT, etc.) to have "secondary meaning" of source in consumer's minds. In other words, they'll stop thinking "this is an Apple Computer from Apple Inc." and start thinking "this is an Apple -- another word for any white, smooth, and shiny computer."
This is one way trademark rights can be lost.