While there might be a semantic difference, the question is whether it is a valuable one.  What value is there in having a MSG_DONE flag that only implies "end of message"?

But then I'm not sure what you do with a sequence of frames that form a message, if you aren't concatenating them.  That's certainly the desired behavior for web sockets (though it's not strictly concatenation in a practical sense, it's more like it's adding those bytes to the message stream).