The typical wrestling crouch may be an effective defensive stance against the single leg take down, but it would be a horrible posture against the fixed bayonet.
As a grappling art judo is unique in that it favors a natural upright posture over a crouching (jigotai) stance. From a pure wrestling competition standpoint an upright posture would be ill advised, but from a combat and self-defense standpoint it is the best stance to deal with the variety of ways an enemy or assailant could launch an attack. Remember that in combat and on the street there are no rules, so an attack can come in the form of a fist, a single leg take down, a swung bottle, a thrown rock or a bayonet at the end of a rifle. The typical wrestling crouch may be an effective defensive stance against the single leg take down, but it would be a horrible posture against the fixed bayonet. Shizentai, the natural upright posture, is the only posture that gives the defendant the maneuverability he or she needs to deal with a multitude of attacking angles and forms.