Click To Play breaks whenever a 1px Flash embed is used - you can't click what you can't see. I generally whitelist sites I trust (YouTube, Vimeo, Google Maps, ETrade etc.) and never have to click on Flash from those sites. Other sites show me Click To Play and 90% of the time, it's ads I want to ignore or the single video link I want to click.
That icon doesn't appear on all sites. Sometimes it seems like the site does feature detection, comes to the conclusion you don't have flash, and refuses to serve anything useful.
Also on OS X the Flash mouse capture is broken if you enable click to play in Firefox.
