You should not use time travel when thinking about international relations, either. The question is about what the US chose in 1971. The Soviets weren't in Afghanistan until 1979.
That doesn't change the fact that exactly the reason that the US backed Pakistan was as a foothold near Central Asia to counter Soviet regional influence. The particular time and place at which that later became useful, obviously, was not predicted in detail at the time, but the concept of the need was.
Now, whether the cost of supporting that regime is justified by either its actual utility or the reasonably anticipated utility at the time the decision was made are different questions, but you can't really fully discuss the decision without considering its actual rationale.
But at that time, the US had both India and Iran as friends. Having Pakistan as a friend to plug the gap between them seems like a less important goal, especially if it comes at the price of alienating India.
The thinking may have been something like: India isn't going to go into the Communist orbit even if we offend it, because of their border trouble with China. But Pakistan might. So it's more important that we keep Pakistan as a friend than to keep China.
Na, US didn't have India as friends. Specially because India started the Non Aligned Movement (basically we aren't choosing sides, we'll have relations with both US and Soviet)
Americans had the attitude that if you aren't with us, then you are against us. Plus India had a socialist economy
Now, whether the cost of supporting that regime is justified by either its actual utility or the reasonably anticipated utility at the time the decision was made are different questions, but you can't really fully discuss the decision without considering its actual rationale.