As the Connection String Spec states "... Keys that aren't supported by a driver MUST be ignored. A WARN level logging message MUST be issued for unsupported keys. ... and "... Any invalid Values for a given key MUST be ignored and MUST log a WARN level message. ...", any invalid configuration options would only ever be surfaced client-side.

By communicating the URI Option Keys and URI Option Values during the driver handshake, this information would be available in the mongos and mongod logs. This would support teams understand what options (both valid and invalid) are being configured, as well as how prevalent certain configurations are in the field.