conn_throttle.so is a shared library used as a callout from the MTA's mapping table that uses an in-memory table of incoming connections to determine when a particular IP address has recently connected too often and should be turned away for awhile. While having an in-memory table is good for performance, its largest cost is that each individual process on each server maintains its own table.
In most cases, the conn_throttle.so callout is done in the PORT_ACCESS mapping that is accessed by the Dispatcher, a single process on each system. The only cost is that there is a separate table per server.
The primary improvement by MeterMaid is that it maintains a single repository of the throttling information that can be accessed by all systems and processes within the Messaging Server environment. It continues to maintain an in-memory database to store this data to maximize performance. Restarting MeterMaid will lose all information previously stored, but since the data is typically very short lived, the cost of such a restart (done infrequently) is very low.