Per the discussion in drivers, https://jira.mongodb.org/browse/RUBY-1647 covers non-timeout errors but does not cover timeouts. Consider the following case:

• Client sets socket timeout to 1 second
• Client sends a query which takes 100 seconds to execute
• Client attempts to read the response, and should time out after 1 second
• Is the connection now returned to the pool and considered available and 100% functional?

Such a case can make a non-monitoring connection perpetually time out while monitoring connections are operating properly, there is a working topology and the topology is not changing. But a particular connection that a driver may be trying to use could be unusable for extended periods of time.