Even if unusable, they have the network effects and brand... just get something out there to play with that will make some sort of impression, however flawed, that can quickly be iterated on. It can be ancillary to the current experience.
Bing suffers from being so historically unpopular that from a branding perspective it's synonymous with failure. It's meme territory. It's not like MS is going to overcome that in a matter of weeks or even months.
Google has plenty of rope here.