Question is why are we investing so much in submarines?
Not that I'm against it but what's the logic behind this?
Pakistan has always thought along the lines of what capability we can acquire for the dollar being spent.
Pakistan was the first country in South Asia to acquire submarines, because,
They are harder to detect, track and therefore destroy. It can be used more effectively in attack or defence then a surface ship.
In attack, you have a lone submarine roaming the Arabian sea, and the Indian navy has to deploy lot of resources to look for it, and after having found it, it is harder still to destroy it.
In defence, the resource allocated by the Indian navy to search for the submarine cannot be used to attack Pakistani shipping. Plus it forces the Indian navy to keep its big assets hidden, or out of harms way, incase they face a sudden attack. It keeps the waters clear, for fear of that submarine.
That's the value of a single submarine, now if you have lets say a single frigate or even a destroyer,
it is on the surface, easily discovered, with its larger force and aircraft carrier, India can easily destroy it.
So that's the deference between using a single submarine or a single destroyer,
now when you have multiples, you change the scenario altogether, strategy and whole host of other factors come into play. This is just a simple explanation, but it catches the value of a submarine.
Now with nuclear weapons, it adds another factor to Pakistan's strategic capability.