as someone who already has an mba, i'd say don't do it unless you are really interested in business. it will make you more marketable to some practices, but most really don't care. so, it all depends on what you want to do in the long run. personally, i love practicing anesthesia. i intend to concentrate on doing this over the next several years. probably, at some point as i become more senior in private practice, my business training will come in handy. but, you will equally have time to get a business degree once you're out practicing if you want.
bottom line: will it help? maybe but probably not. will it hurt? absolutely not. more training, no matter what it is and if it's applicable to your field, is always a good thing. but, most people would rather spend their time doing other things. fact is, if you have an mba you'll likely get relegated to some part of managing a practice, something you may not have a particular interest in. so, don't do it just to do it or out of fear. you'll be miserable while you're in night school doing something that doesn't really interest you instead of being with your friends/family.