Oddly enough, back in my mis-spent youth, I actually did some work for a major testing center, where it was my job to perform fire tests, and alarm systems. Tests were both limited 'bench' tests, as well as full-scale fires.
The real 'fly in the ointment' for smoke alarms is that pesky detail that most fires smoulder forever, before bursting into flames. The riddle is made more complex by the fact that the most common detectors are not particularily sensitive to "cold" smoke.
Anyway, in the full-scale tests, it was quite easy to see the smoke form a cloud near the top of the room. Any indentations, or projections, would have a visibly 'clear' area near them... imagine the cloud of smoke as a giant cotton ball! Even after the cloud was quite thick, there remained clear areas in peaks and corners.
The instructions are based upon these tests, and are intended to make the detector as effective as possible. That these are often places that are nearly impossible for the occupant to access is not one of the things the code considered.
The issue is right up there with lights placed were you'll never be able to change a bulb.
There are other solutions out there... light beams being but one... but these solutions will cost far more than the common $4 detector!
