Some of the kindest and most sincere teachers of asanas, I have ever met, belong to this system of practice. In spite of the many benefits of this style of Hatha Yoga practice described in these slides from a workshop, due to the way it is taught by some teachers, it unfortunately, deters many practitioners. This is not just true of the Ashtanga Vinyasa school, but other schools of advanced asana practice too. The purpose of the article is to point out some of the ways advanced asana practice should and should not be taught, because such powerful techniques need a strong scriptural grounding and the reason for doing complex poses should clearly be told to participants, so they understand the benefits and risks. Yoga asanas, after all, were developed as a spiritual practice to still the mind, primarily, and therefore should use scriptures as a guide. [Ref: Know the rules of yoga well to break them correctly, Removing the shroud of mystery surrounding ashtanga yoga]. 

Guruji Pattabhi Jois, would say that some of the poses in the primary series, which is called Yoga Chikitsa or yoga therapy, are the most beneficial for health. If it is just for health reasons, one need not do extreme poses. However, to go deeper into states of relaxation, as Patanjali says, the effort needs to be higher too within reason. Prayatna Shaitilya anantha Samapattibhyam - Effort followed by relaxation results in infinite bliss. The prefix "Pra" denotes appropriate and "Yatna" denotes effort. This is where advanced poses come in (Ref: Entering the eye of the storm). 

The Ashtanga Yoga primary series of poses would seem like a never ending ordeal, especially for a new practitioner. Just when one would start to feel that the poses in the sequences had gotten easier, there would come a grueling pose that would break the reverie. The last pose in the Ashtanga yoga primary series (after the so called 'cooling down' poses) is a pose called Uthpluthihi. It is an arm balance with the legs crosssed in padmasana. It is considered a difficult pose to hold, yet considered an important pose for health reasons because it builds core strength and it also exercises the internal organs in the abdominal region through this core action which is exercised using the Uddhiana and Moola Bandhas. Due to these benefits, some yoga teachers will require their students to hold this pose for a full minute, which is not an easy task, even for an advanced student.  

Some teachers take the pleasure of teasing (or tormenting) their students, when they are holding this pose, by elongating the count when the count reaches nine, when the students would already be huffing and puffing and beginning to hold their breath. Does this sound familiar ..,7,8,9,9½,9¾,......,(and finally after a long pause) 10 ?

Jokes and teasing aside, forms of yoga like bikram yoga, hot yoga, power yoga and ashtanga yoga are sometimes taught and practiced in ways that are "un-yogic". The insidious aspect about some of these schools of yoga is that it is taught by some teachers under the cloak of ancient tradition, out of complete ignorance of the many scriptural aspects of yoga. The Vasanas (literally means smell or metaphorically the energy exuded) of this kind of harsh practice attracts similar minded people who are willing to go to great lengths to punish their bodies (and that of their students once they become teachers) and sculpt muscles.

For these reasons, these schools are perceived in poor light by many. Some well respected researchers on yoga, like Alistair Shearer, the author of "The story of Yoga" have used the words "Spandex body building" to describe these practices. 

Scriptural Interpretations

There are two great scriptural texts on yoga. One is the Yoga Sutras of Patanjali and the other is The Bhagavad Gita. The Bhagavad Gita (17.5-17.6) clearly outlines what is considered a demonic form of austerity (yogic practice). Krishna uses the word "Ashastravihitam Ghoram", which means "not enjoined by the scriptures and gory" (the  english word gory and the sanskrit word "ghoram" seem to have the same root), to denote these forms of austerity.

अशास्त्रविहितं घोरं तप्यन्ते ये तपो जनाः ।

दम्भाहङ्कारसंयुक्ताः कामरागबलान्विताः ॥ १७-५॥

कर्षयन्तः शरीरस्थं भूतग्राममचेतसः ।

मां चैवान्तःशरीरस्थं तान्विद्ध्यासुरनिश्चयान् ॥ १७-६॥

Those men who practise terrible austerities, not enjoined by the scriptures, given to hypocrisy and egoism, impelled by the force of lust and attachment . . .

Senselessly torturing all the elements in the body, and Me also who dwells within the body — you may know these to be of demoniacal resolves.

Lord Krishna tells Arjuna that those who subject their bodies to such harsh austerities, not only torture their bodies, but also him, who resides in every being.

The fine line between ecstacy and torment

There is a fine line between ecstacy and torment in asana practice. When practiced with the intention of tuning into the Prana, one finds ecstacy. When practiced with the intention of body building, one loses association with the prana flowing in the body; this results in torture.

Guruji Pattabhi Jois when asked the purpose of each series of ashtanga yoga , would say, that the primary series mainly serves as "Yoga Chikitsa", treatment of the body; the secondary series' main purpose is "Nadi Shodhana", cleansing of the Nadis; he would then, when prodded about the benefit of the advanced series, say, jokingly, that the advanced asanas are used mainly for demonstration. 

References:

1)  The Eustress and Distress of Advanced Asana Practice: A Scriptural Perspective

2) Removing the shroud of mystery surrounding Ashtanga Yoga 

3) Knowing the rules of yoga well to break them correctly

4) As the body is for the Purusha, so are asanas for yoga