To summarize. . .

Testing for max HR is extremely difficult, potentially dangerous, inaccurate and unreliable outside of a laboratory and, generally, unnecessary.

Testing for anerobic threshhold is more easily done outside a laboratory than testing for MHR, so, therefore it will be more accurate for the average person (assuming he/she understands what AT is and how to arrive at it).

Since AT is more easily determined, it is easier to establish training zones and to do so with more confidence.