I'd guess that there is a huge number of such flights being operated and those operators who stick to the rules are being checked all the time...
Hmmm, everytime I was checked by the SAFA dudes, they checked others as well. I have heard also a lot about what the findings by SAFA checkers are, but none ever told me "we pulled a dude without the required licence/rating/medical off a jet the other day". not that this would mean anything, but I personally wouldn´t like to risk it. Hell, I sweated heavily when I discovered that I left my medical at home once - and to get access to a copie isnt that hard. OTOH I´m sort of a pussy, I would not dare to fly an A/C as Captain for which I hold a SIC rating only...