No reason it can't do both. Critical Hit charts are unlikely to ever be balanced. So, they make a good module for those who want that kind of thing in the game without effecting the underlying math overly much.I vastly prefer crits and critical misses to cause effects of some kind, than max damage. Not even close. The problem with making it a module is that max damage will be in the math.....
It seems to me that maximizing damage is a pretty dull way of perceiving crits in D&D?
It should be strictly a modular option
Have you seen Paizo's Critical Hit deck? Each card has four options based on weapon type.I think a deck of cards would be more fun than a chart. Cards would represent opportunities that striker takes when dealing a critical hit. For example, a fighter striking a critical hit draws 3 cards and chooses one. Then applies its effects to his attack. Each card would say what extra effect the hit does and how much extra damage.
Two decks, one for weapon attacks and the other for spells, spell-like attacks etc.