Different martial arts approach the fighting stance from different angles.  Some are meant to keep the opponents at a distance, some are meant to allow for quick defense.  All incorporate some level of the following things into the stance to allow for quick defense either from footwork, angling, of parries and/or blocks with the hands.   A lot of these stances are, however, meant for hand to hand combat.  When you bring weapons into the equation, especially bladed weapons, these stances have several weaknesses, especially when you move away from the traditional weapons practice of thrust and defend.  On the street, you may not even be aware that your opponent has a weapon.  Take for example, the follow stance. 
While this stance may be perfectly effective in defending against a punch or a kick, the first thing I think of when I see this stance is that:

1.  the elbows are in providing protection to the vital organs from the front such as the heart, lungs and liver. 

Any confrontation with a bladed weapon is going to be quick, complex and messy.  No stance is going to ensure that I don’t get hurt.  Footwork and parrying will help, but asI’ve heard many times–if a blade is involved, you’re going to get cut.  However, at least here there is a fighting chance of ensuring that the first strike is not one that damages a vital organ.