It's so when someone trips over it and breaks it off you can dig the foam out around it and get a coupling back on it easily......
JAP>
Would seem to fit my experiences better, I've never seen a damaged conduit that was poured in the slab. In fact we are specifically allowed to embed most solid walled raceways in concrete by the applicable chapter 3 article on each raceway type. ENT is even allowed to be embedded - if anything is weak and going to succumb to such forces it may be ENT.
Breaking off at the surface - I have started setting a coupling flush/ slightly below finished surface, the concrete finishers like it, it ends up allowing nicer looking finish most of the time, and you don't get other trades knocking them over and breaking them before you get there to finish the run either. Trick to finding later - tie a cable tie to the conduit - leave about an inch of the tail end of the tie sticking above the finished surface, power trowels just bend it over when they pass over it, but it sticks out like a flagpole after finished and is easy to find.