The Arch of the Hips

I have been reading a lot about the hips, how they move and what types of movements tend to cause instability (including this article by Michaelle Edwards).

It is well known that the legs and hips from an arch, a stable way to transfer the weight of our torsos down through our legs into the ground. The sacrum acts as the keystone to the arch. It is the point where the force is the greatest and where it splits down the legs. 

This has me wondering why we encourage people to do any yoga postures with their feet together. Does this compromise the strength and integrity of the arch? Isn't it important for the feet of an arch (and therefore the feet of a human) to be sufficiently wide to make the transfer of weight most effective? Or does standing with feet together only make balance more challenging with no effect on the sacrum? 

I am going to keep these questions in my mind as I practice for the next few weeks.