Pros, cons and the actual need.
If I am not wrong, canister is needed if missile is built of composite materials as they don't behave well if exposed to elements.
India needs to canister their Agni as the missile has composite body.
Pakistan uses steel tubes and a regular coat of paint is enough to keep the integrity of the system.
Likewise the saying "Putting all your eggs in one basket".
MIRV Missile will have multiple warheads in one location. One system gets hit , causing a significant dent in country's strategic capability.
Warheads distributed over multiple Missiles, will be difficult to track and attack .