This has actually become fascinating to consider.
True, the option is [M]odify and not [M]aximise, even though identical destinations could be achieved. I recall reading back somewhere in one of the game manuals (not -proof- but rather a roadsign pointing to the intention) of the modify button to be a means to customise your game pc to match a tabletop pc. Alas for laziness, I have not picked over docs to find its mention...
From a design standpoint, I've never really considered what the [M] could mean. Sure, difficulty settings could be ranged and (as Two-Bit Termite's latest offering poses) managed atop artificially, but I've never personally considered what a fully modified party could mean to the balance of play... I would assume, at best, a few additional spells, better saves, taking extra hits before the ol' cross-eyes cross-bones, tohit bonus, damage bonus, ac bonus... actually, now that I've typed it, it does seem abundant.
Is there parity here with a modified in-game difficulty setting? Is there an ethical question involved? Are your characters really yours anymore if you've 18'nd your way to the fore?
I'll have to consult a Temple of Tempus. In the meantime, I will consider this further. To leave with a parting thought: none, and I do mean none, of the npcs in the history of the games have ever come stacked and packed, 18'nd and... weaned? Weaned. However, that noted, pcs are mentioned in sourcebooks (as I can recall) as being heroic specimens of their numbers: meant to be pcs above npc/commoner status by dint of their exceptional stats.