I use /highlight when I'm in a crowded place. I have nothing against using slang in your RP, I do it with close friends to give it a funny twist, to make it a bit more interesting. However, when people roleplay doing an action, e.g stepping out of the car and waving with their hands, some people incorporate their speech into the /me. E.g /me steps out of the car using his right foot first, whilst waving towards John and saying "Hi".
I don't get the logic of why you would need to mix actual IC chat with your IC actions. Is it easier? Only if you're lazy, though it's just as easy to use the actual function to say Hi.
Does it bother me? Not really, does it pinch my eyes when I have to snap out of my interaction with others? Yeah.
Will I say something about it when I'm playing with them or after? Nah, it's a form of respect and leaving people with their dignity.