from Kala Sarpa Yoga, Why Such Fright? - preface
No ancient standard astrological text has mentioned Kala Sarpa Yoga (literally the 'Serpent of Time' - all planets between Rahu and Ketu). If KSY was so important then why did not the great [sages] Parashara, Varahamihira and others, talk about it and stress it as a yoga (combination of planets) completely ruining a horoscope? In fact, Rahu and Ketu do not figure in the well-known yogas, except Guru Chandala Yoga, and Ashta Lakshmi Yogaswhich too have been derived from some other books and traditions. There are some traditional astrologers who do not even show Rahu and Ketu in the varga charts (divisional charts), saying that the chaya grahas (shadowy planets, Rahu and Ketu) have no role in varga charts. I have not found that correct, but it shows why Rahu/Ketu have not been such an importance in the formation of yogas.
If you removed Kala Sarpa Yoga and Sadhe Sati (Saturn's transit over the Moon) from astrological parlance, many astrologers would lose their source of money. These two are their standard explanation for any difficulty and the next step is to demand money for shanti or propitiation of planets and/or purchase of gems which is a rich source of their fortunes.
reprinted with permission