Re: why do people outside Aikido for IS?
Posture (shi sei) is good when it is harmony with your every movement. It is not an isolated aspect of Aikido. There moments in some techniques where posture is a more important ingredient than another, but always present. Same for ma ai, kimusubi, kino nagare. It is about finding the right mix for a given situation: harmony.
Like I said: people venture outside Aikido because they might not know what they are looking for. perhaps your technique can be improved by better shi sei, but if you do not know that, if your teacher does not point that out...