I will side with the illegal taxis, I have many friends doing this business, always polite and on time and never overcharge me, in fact many times they just say give me what you think is a fair price.
My experience with Karwa is that they don't use the meter, charge double what the actual cost should be, most of them don't know their way around Qatar you have to tell them where you want to go and then they pocket the money making it a very good business for themselves.
Private / illegal taxis will win the day until somebody decides to run Karwa in a professional and efficient manor.