We end up completely misunderstanding the purpose of asana practice, which lead us to be attached to reach something that is outside... subconsciously we think that we are advance because we are more flexible, and we love that word: "advance". But flexibility has nothing to do with being advance. Actually, from my perspective, I rather to be beginner always, because then I can keep learning.
When we practice hip-openings, it is not because we have to put the leg behind the head and performing fantastic vinyasas, though is fun to do it, but it has a purpose.
When we open our hips beyond the limits we have created as a product of our lifestyle, then there is something very important and subtle happening on the body, that most of the times we are not even paying attention. When the hips are opened, they are released from tension, so the pelvis can be perpendicular to the floor and the spine can be completely extended.
But what is the difference between "just sitting with the back straight" and "sitting with the back straight with the hip flexors relaxed"?? The answer is given by yoga sutra 2.46: "sthira sukham asanam" (asana must be comfortable and steady). When the hip flexors are released, the thighs are also released. My teacher Ushaji, says that if the thighs are tense, the mind is tense. So, when we seat, we tend to tense the hip flexors and as a consequence we tense the thighs, which is a reflexion of the state of the mind and viceversa. That is the difference, when the hips are open, we seat comfortable and stable.
By opening the hips we create the space for the thighs to relax and we prepare the land for the mind to be quite. Does this remind you of something?
Yoga Sutras 1.2 "Yoga Chitta Vritti Nirodhah". Yoga is the cease of the fluctuations of the mind.