The last time that India was defeated in a home series was in 1987-88, by Pakistan. Look at India's performances abroad, and one sees almost a mirror image of the domestic performances. For the record, India has won series in New Zealand in 1967-68, in the West Indies in 1971, in England in 1971 and 1987, and in Sri Lanka in 1994. To paraphrase Tiger Pataudi, tigers at home turn into chickens abroad. First, the entertainment value.
Variety is an essential part of entertainment in cricket.
Playing every Test on slow turners is every bit as boring as playing every Test on fast bouncy wickets or on seaming green-tops. In the '90s, India has not played a home Test in which they played less than 3 spinners. Pace bowling at its finest has not been seen in India since Marshall and the West Indies toured in 1982-83. The Australian wickets in the '80s epitomized this notion of variety. Regardless of who was touring, Sydney helped the spinners and Perth was a fast track.