There are some situations where it is difficult to pass in a client option, and it would be preferable to be able to set sslInvalidHostnameAllowed as a parameter on the connection string.
For example, the Mongo-Hadoop connector does currently support setting client options. (I have also written up a request for Mongo-Hadoop to support setting client options [HADOOP-245], but from an API perspective it would be more easily handled on the connection string).