First, let's understand why RBI intervenes to defend the rupee in the first place.
A weak currency worsens our fiscal deficit, fuels inflation and slows down international trade.
Clearly, RBI has a mandate to improve the situation on behalf of the GoI and the citizens of India.
Forex market is highly volatile and a breeding ground for speculators. This is why the FX market is the most liquid in the world with around $7.5 trillion (with a T!) in daily turnover.
So, RBI has to control speculation in the USDINR segment to ensure stability of the Rupee.