I don't mind people using brb, even if it's done incorrectly. Like if they're on the game but using skype, their character being afk the entire time. Just means they want to communicate through a different, perhaps slightly more private chat that doesn't hold so much to me.
But the whole 'brb' when taken into grammatical consideration doesn't make sense. Now if you said 'They're afk' (away from keyboard) maybe that'd make more sense, but people saying 'They're be right back' doesn't bother me. Simply because 'brb' is a commonly used internet term for short-termed or long-termed (even if 'be right back' doesn't exactly mean a long absence) and a lot of people use it. It's grammatically incorrect, but it's easy to understand, you know?