Soon to be Med School grad. 200k loans. I don't understand loans. Please help.

Hey all, Long time lurker here and I can't seem to find a straight answer on my question anywhere. If this has already been answered a billion times I am sorry (I did my best to search for it!). Anyways, as I said I will be graduating with a little over $200,000 in loans mostly at 6.8%. I'm going into a 5 year residency with likely 1-2 years of fellowship afterwards. My question is pretty simple, why the hell would I NOT do IBR? Or maybe more simply, why would anyone not do IBR? As I see it (I'm probably seeing it wrong), with IBR I have a minimum payment that is way less than the standard fixed 10 year. HOWEVER, say I live with a roommate/gf in a city that is cheap and can afford AT LEAST if not MORE than the standard fixed 10 year, I can just pay that and get done paying my loans quicker, right? Wouldn't the same hold true for anyone in any situation? IBR gives the safety net to pay less, but if you want to pay more, you can. What am I missing?