Drs who 'over service' are investigated. I had a GP a few years back who worked crazy hours in a small practice in an area that was short on Drs, she was investigated for over servicing and was told she couldn't take on any new patients.
Generally extra tests are performed by a path lab or imaging lab that isn't directly associated with the originating Drs practice - so they see no additional income from referring you beyond you going back to get the results.
I was diagnosed with cancer last year because of a Dr ordering what might have been considered an extra test (I needed a blood test because of some medication I was on, she decided to do a full work up because it seemed like a good idea at the time). If my GP had not done the extra test, it's likely I wouldn't have been diagnosed so early, would have gotten much sicker than I was and would have required chemo in addition to surgery.
Given the current standard HR policy of requiring a Drs certificate is you have a sick day, I would say most of my Drs visits these days are to get a note saying 'yes she really was sick' when I have a bad head cold or a migraine. I would be better off at home resting, but have to drag myself out to get a bit of paper and to be told to go home, rest, stay warm and keep up my fluids - all things I already know. Most Drs don't have time for unnecessary tests, they're too busy issuing repeat scripts and medical certs.