Should PPL (and microlight and glider ect ect) training include teaching people "if you're x DME from the threshold don't cross the instrument approach between y00' and z00'"? - I'm sure it doesn't for most people at present, certainly I was never taught that.
This is my take on it....
IAPs vary in design, and one airfield may have a GPS, NDB, VOR, ILS, LOC, etc...approach designed for it, and many of these have different descent profiles or approach paths (3 degree, dive and drive, 30 deg off runway heading etc...). They also may / will have outbound procedure turns to add to the mix. I think it would be good to give PPL students an "appreciation" of IAPs and where you could expect to see traffic arriving from.
However, in this instance I don't think the microlights showed a lack of airmanship. They were crossing a IAP path in VMC, outside CAS, and as you know from your training, and as your instructer showed, during an IAP in VMC then the commander has an obligation to see and avoid, as per normal VFR, despite possibly being on an IFR flight plan (which is the reason you cannot practice IAPs solo). The micro's could reasonably expect in this case then that you WOULD be looking out of the window as they should have been. It would have been bad airmanship to be doing the same just below a cloud base of course, with the chance of an aircraft on the IAP popping out with no time to see and avoid.
My own view is that the safest place to cross an airport is overhead, above any circuit height and above any "overhead join" height if possible. I'll even plan cross countries to use airfields as waypoints but make sure I am well above them. It serves two purposes, they are easy to spot and IF something goes wrong, it is very easy to divert....