Sure they did, usually they in many instances cover up things for each other.
Maybe not.
It is true that many officers will use "discretion" when pulling over a fellow officer for speeding etc. This would be the same sort of discretion that I used many times when on motorcycles to give someone a warning instead of a ticket. In fact, there was pressure to write tags, not a quota per se, but there was an understanding that you should average a certain number of tickets. So even though I enjoyed riding, I asked to get off motors. I felt badly giving some people tickets when I would have preferred to give them a warning - you can tell when a warning is all that's needed to do the job, and when a ticket is the better option.
Back to dealing with other cops. Sure there's camaraderie, team spirit, sometimes a subdued "us against them" mentality, thin blue line, always help another cop in need, it is a brotherhood. BUT, and this is important, you don't turn your back on criminal activity, ever. And you don't cut a drunk cop loose to drive home, and you do contact your Sargent or his if he's on a different department. You can't condone a DUI cop hurting or killing someone - no way would I let them drive off.
How would you eliminate "professional courtesy" between one cop and another? I don't think it could easily be done, frankly. Just as two friends on an assembly line would treat and great each other, there is a defference that comes with the territory. It's a tough job, I lost friends, one a rookie who I'd cut loose a year and a half before (I was a training officer), there's a very natural tendency to stick together. When I left the department to devote all my time to a company I founded, what I missed most was the camaraderie. My wife didn't at all miss worrying about me every time I drove off to work.