then you don't understand the Trident warheads capabilities.
each warhead has a 'dial-a-yeild', this means that while each warhead can explode with a force of about 500kt, they can also explode with a force of 0.2kt, 5kt, 15 - 20kt and about 100kt.
its called 'intelligent targetting' - mainly because TridentD5 isn't a weapon designed to destroy cities (which it can also do) but as a 'counter-force' weapon. it was designed to attack soviet ground launched ICBM's.
the spacing of hardened ground silos means that one weapon (almost regardless of size) is unlikely to destroy more than one silo, so you need one warhead per silo. if you used a large warhead - say 15kt - it would throw so much debris into the air that it would destroy other incoming warheads so leaving some ICBM silos untouched and therefore able to launch once the debris cloud had subsided.
therefore you use a much smaller warhead on each target timed to arrive en masse. the function of the larger yeilds is to destroy more vunerable systems/assets over a large area (airfields, logistics sites, Naval fleets at sea, ports, docks and communications sites) or against very hard targets like deep underground bunkers.