I personally think it's a kick-ass cool idea. Especially if the swarm has some kind of hive mind and can telepathically communicate with the PCs...and anyone it kills becomes part of the swarm...
i.e...
Swarm Healing, At-Will, Immediate Interrupt
Trigger: When someone damaged by the swarm during this encounter dies.
Effect: The Swarm heals Constitution points of damage.
Secondary Effect: The Swarm gains a level, recharge 5-6.