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.