C17 purchase does raise some reasonable questions: why not IL76? why strategic lift when combat strength is so low? Boeing closed it production line after servicing India's order, are these end of life-cycle products? (Incidentally the artillery gun that US was pushing also closed its production line)
But the two things that I think C17 has going for it is that:
1. If Boeing had used best practices from its civilian aircraft line in designing C17, then it should have higher availability and lesser turn around time (The Airbus refuelers might be a good idea for the same reason. In their case they use the exact same fuselage with blacked out passenger windows). Ilyusin's civilian projects were not that successful.
2. The C17 is the only modern looking ac in the IAF fleet (that and Druv). Even the most gorgeous fighter, Su30MKI, looks a bit long in the tooth. A pic of any ac with the C17 in Indian colours in the background still makes it look good. Let them have it, atleast until they induct the Tejas