|The fact that I use this in the post does not mean it is not an effective stance.|
If it is true that stances are transitory; if it is true that the fundamental principles of martial systems specifically "economic motion" so techniques and all their perts must meet a most economical principle than deep stances are not adequate in actual life application.
It true then the question of purpose becomes narrow. Deep stances are simply the result of pushing karate into a watered down physical fitness regimen for school aged young adults. When they pushed the system into the schools things like bunkai and technique were modified and stances were deepened to create a greater physical demand on the practitioners.
If you have to drop into a deep stance to apply a technique and then move out of that stance for the next encounter then your first adding more time to reach the deeper stance; your transition takes longer and thus affects the actual applied technique; your technique uses up energy as well as the time and effort taken to reach a deep stance; your technique then loses that energy which stays in the body and since deep stances must first be assumed and rooted to work you lose power to the technique.
Now, we have only half of the equation. The other half is the time, distance and energy used to get out of that deep stance so you can transition to another appropriate deep stance to apply another technique.
All of this telegraphs intent in a real fight and if this is a predatory violent attack you are going to be way to busy trying to figure out what deep stance to take to fight back - good luck with that one. Lets not forget that the assumption of deep stances to time to application of technique means an adjustment to the the technique causing it to take longer thus telegraphing.
Deep stances server the purpose of strengthening the legs, teaching body mechanics such as alignment, etc. and they also get the practitioner to learn the principles of the system BUT once that initial foundation is laid a practitioner must move beyond the fundamental basics such as deep stances.
Take a look at the Okinawan styles the use deep stances and watch what happens to the more senior karate-ka, the stances in kumite seem to disappear for the more advantageous transitory natural stances necessary for effective application of karate techniques.
Make sense? Yes? No? Maybe?