The word 'should' rather than 'must' is used in all AMCs since there may be a number of alternative AMCs from which you could choose in order to comply with the requirement; it does not mean you can choose to ignore the requirement.
It is not the word 'should' that is significant in this case but the word 'can'. The AMC states that the amount of training 'should' be determined on a case-to-case basis by the ATO taking into account a number of factors, including the time lapsed since the expiry of the rating (fair enough). It goes on to state that, when determining the needs of the pilot, "the following items can be taken into consideration" - not 'must', not 'should' but 'can'. The clear inference is that the ATO may also determine the needs of the pilot without taking into account the suggestions in paragraph 3 of the AMC.
A classic example is the case of a pilot who has allowed his MEP class rating to expire by more than 3 years but has been flying small MET aeroplanes (e.g. BN2T) regularly. It is unlikely to be necessary for such a pilot "to again undergo the training for the initial issue of the rating" in order to "reach the level of proficiency necessary to safely operate an MEP aeroplane". Indeed, it might well be that the ATO decides that no flight training is necessary at all, although experience indicates that it is more likely that an ATO will over-estimate the training required, purely to generate revenue.
In short, the determination of the amount of training (if any) required is entirely at the discretion of the ATO irrespective of the suggestions contained in the AMC.