I believe it stays typed up in the conversation so that it's there when you get back. You can also copy the text and paste it into a note so that you could use it later.
What do you mean? You type a message in the Messages app in a conversation there and then leave and come back and it's still there. Is this for an existing conversation or a new one?
I'm not sure something like "careless" would really apply here. It's just a function that Apple didn't include, like many other functions that aren't necessarily there in many other apps from many other developers. No one really covers everything that someone wants. Sometimes some options aren't typical or sometimes some implement an options while others don't.
As far as a drafts for messages, I'm not sure how typical of a feature that is for most messenger apps (as opposed to email clients for example).