First of all, I must say I am intrigued by the depth of knowledge, especially on the history and literature of Taiji, of the contributors to this forum. Amazing!
Due to our physical differences (height, weight, flexibility, age, state of health, etc) we practise the Taiji forms differently. The important thing is that the principles of Taiji must not be too far off. All these different practises are just the means to the ends.
One of the advantages of dingshi practices is that one has to be totally relaxed (or "song")to hold these positions without your joints complaining ever so strongly; and once we are totally relaxed our qi can flow smoothly.
Once totally relaxed, one may find that the posture will adjust itself to allow maximum efficiency based on his/her own physical characteristics. And that further relax the body/mind, and thus attaining even more efficiency, and the cycle goes on.
Hope it helps.