My friends who say not caste but only economic backwardness should be the only criterion for reservation, perhaps do not know anything about the reservations.
Reservations found their way into Indian constitution as a result of Poona Pact, 1932 signed between Gandhi and Ambedkar which formed the basis for reservations to untouchable castes and tribals. While untouchability was exclusive to Hindu or Indian origin religions, tribes were based on their ethnicity and areas they lived. Thats the reason that when a tribal converts to any religion he retains his constitutional reservation status but when an untouchable converts to a religion on non-indian origin his social status changes in his own community.
A tribal or an untouchable may be a millionnaire or a billionnaire but the outlook of the so called civilized and high caste people does not change towards them. It is the reason why reservations are not valid on economic grounds. They face discrimination because of their social status regardless of their economic or educational status. Mandal Commission, however, recommended reservations for economically and educationally backward castes. The commission found a pattern in economic and educational backwardness of certain castes and recommended corrective measures. This backwardness had nothing to do with religious affiliation but universal and thats why certain muslims are also part of OBC reservation. While Islam and Christianity assure socio-religious equality among its followers, the same can not be assured for economic and education inequality.
In my personal opinion economic and educational backwardness for reservations can not be considered unless the religion, caste or ethnicity was responsible for this backwardness and in that case the basis for reservations become caste, religion or ethnicity. Today Parsis in India are the richest and forward community but yet they are provided with special privileges to increase their population. In this they will be at par with the Andaman tribes on the verge of extinction. A special constitutional privilege shoud be viewed with a specific perspective and not generalized.