It's something I have to train out of people often. I see a lot of people doing it, and there is an idea amongst some coaches that the flared elbows are some how to make kicks easier to catch. For me though, if it does/doesn't make it easier to catch a kick, I would still rather my fighter hit them while they're on one leg as opposed to catch. And you don't NEED those flared elbows to be able to catch a kick, but you do need a tighter stance in order to have quicker more powerful punches. So if I have someone that flares their elbows a lot, I will try getting their arms in tighter (but not so tight that they'll be kicked in the arms all day). Ideally so they can not only go on offence, but use a cross guard and mummy guard easier too! But that's also a part of my relationship with a fighter I train, I'd advise you NOT to do it personally, but at the end of the day you're not working with me, and for sake of argument your coach might have a plan or style that values things I don't.