You're thinking with an MMA/Boxing/sparring mindset. In reality you will want to have at least one hand held low to protect your groin. If you approach me with two arms up protecting your chin, I will kick you in the dick every time before you even get into punching range. Even with MMA rules, people like Connor McGregor and Michael Venom Page dominate their competition without needing to slave to this always "keep your hands up" mentality: