(1) BMDs were envisioned as offensive weapons. They were not supposed to take whole burnt of Soviet nuclear attack, but were supposed to shield against an attenuated counter-attack after USA have done a counterforce strike on Soviet nukes. It is not just limited to BMDs. BMDs, SSBNs, and MIRVs; all were designed in order to hit opponent before he could get the wind of incoming strike or to survive a attenuated one under BMD shield.
(2) If you want to win a nuclear war (rather than get mutually annihilated), you need technologies like BMD. With the kind of arsenal that Pakistan could field (due to limits of fissile material with Pakistan), India could field a credible BMD. It is not like Pakistan could produce 80,000 nukes like USSR (Which had access to Uranium mines of Kazakastan). Maximum nukes that Pakistan could field is between 190-210 based on availability of fissile material to Pakistan.
(3) Every nuke that a BMD stops is the one that would not be falling on you.
(4) BMD (a demonstrably potent one) would by itself have deep psychological impact on enemy, even if does not live up to its claim. With a deployed BMD, enemy would be circumspect in its activities against you. Pakistan current terrorist behavior is a classical bully "Kya ukkhad lega" behaviour, in which they do terrorist strikes and then hide behind their nukes. A working BMD would make them circumspect of their actions as they do not know how many ,if any, of their missiles would go through Indian BMD.
(5) Nukes have a diminishing value of return as ,after a certain number, having more does not provide you with commensurate benefits. This is the reason why USA and Russia both agreed to reduce their arsenal drastically.
(6) Weapons like BMD, in conjugation with SSBNs and MIRVs, give operational flexibility to GoI, as it could (if it want) try to neutralize Pakistani arsenal, without worrying about what would happen if few Pakistani nukes survive.