yeah and if his glove ends up landing on the apex of a bent elbow you can injure or break his hand. Goodluck if you can 'elbow' his punches.
Ive boxed a come forward guy like this and its very very hard to fight against. Its probably best for smaller fighters but I tend to use it because I dont have very good reflexes some im not very good at slips or timing blocks. Katsidis does this a lot.
This is a blocking technique I use quite frequently with body shots, I kind of brush them aside with the elbow in light sparring but if the guy is coming with mean intentions I am not above jamming the elbow hard in an attempt to discourage further body shots. I occasionally use it for the head shots too but mostly when I raise my arm the forearm catches the shot, very rarely the elbow. In some cases I come forward with my left elbow high during a wide right hook and "accidentally" try to jar the shoulder joint with it as I go into the clinch. I do this quickly and have never been warned but its dirty dirty dirty. I would say most blocking techniques are legal as long as they can tell its not an "elbow strike", which would get you in trouble.
Problem with doing taht is your opening up the rib cage so if somone sees you doing they should throuhg a shot and go hard to the body and they will break your ribs!