How can you say this. Have you even looked at Hangouts?
If this is still missing, it's stupid.
Hangouts is going towards integration with SMS and is supposed to work the same way (i.e. you send a message to someone regardless of their status).
Whether this is a good thing or not, I don't know. But it sort of makes sense in a way. IM seems to be the only form of online messaging where you decide whether to message/call someone based on their status. If you call someone, you have no idea if they are available or not, same with SMS or email.
I also often see the case where a user will perpetually set himself to 'Away' or 'Busy' as he 'never' wants to be messaged, defeating the whole purpose of a messaging service in the first place.