Formations come; formations go. It's like long hair, miniskirts, wide ties, etc. Everything old is new again.
The reasons NOT to mandate formation are simple:
1) Different players; different skill sets.
2) In-game "time and score" situations affect strategies and dictate formations.
As a coach, what you REALLY want is a viable physical and tactical option for every situation. You want to be able to deal with contingencies and "what ifs".
Of course, that takes vision and flexibility, if you get my drift.
