Nothing much has changed geoffers - I did my A2 in 1991 and the same process is in force now.
Pitts - a working category for a military QHI/QFI is B1 - that means you are a solid average instructor who can conduct the whole syllabus in your unit. During the work-up from B2 to B1 you would fly regularly with an A2 instructor (or higher) to assess your progress and refine your instructional technique.
As geoffers said, you have to be recognised as instructing at a higher level and then recommended to attempt an A2 upgrade - it is viewed with a level of trepidation by candidates as it does put your flying ability, instructional ability and aviation knowledge under a microscope and you have to be graded above average across the board.
The purpose of the A2 is to recognise and reward good instructional ability and to create a structure by which senior flying and supervising positions can be filled with a measure of confidence because the candidates have passed a difficult professional assessment. Sadly, no extra pay goes along with it.
A1 recognises exceptional instructional ability and aviation knowledge - there are a few (one of my colleagues recently upgraded) but most A2s have had enough of being checked and examined and not as many go for A1 as might be expected.
The system of checking and mentoring has a proven track record of success and the structured syllabus gives both candidate and examiner the ability to prepare for and assess the test.
Will we see anything like this in commercial aviation? Only in big organisations with their own training setup I suspect.