And in the same time, their competitors (Facebook, who isn't known for being open exactly) can use the same resources to push out more features.
Being open only helps in that users see it as a feature. Right now, not enough users see it as a feature to make being open as a priority for this product. And those who do prioritize openness are already starting to move away from google because it's gotten too big.
Who cares what kind of features others add, if they can't even communicate with each other. Google of course define their priorities. So far they claimed that interoperable protocols are basis of their features. If it's the features they are after and they don't care about interoperable protocols then they aren't better than Facebook and other "competitors" who disrespect their own users by not letting them to communicate with others, as if we are in the stone age of computing, when one couldn't send e-mail from AOL to Compuserve. If that's the case, it should be a good reason to advise others to dump Google services.
They gain advancing interoperable and connected IM network. If they are OK with creating another walled IM mess for selfish interests (as if we don't have enough of these Skypes, Whatsapps and etc.) then let them be boycotted.
Being open only helps in that users see it as a feature. Right now, not enough users see it as a feature to make being open as a priority for this product. And those who do prioritize openness are already starting to move away from google because it's gotten too big.
So google gains what, again?