Janx said:
The 21st century idea of balance is that each PC in the current game session is able to contribute and add value in a fairly equal fashion.
Taking "fairly equal" a bit more flexibly, it was usually the same a few years ago, in the 20th century, Janx! The key difference was that we considered the matter
as players, in deciding which characters would join a given expedition in the first place.
Now, sometimes a higher-level party might take along a notably lower-level character specifically for "seasoning". Equality of contribution is clearly not expected -- but neither is it expected of henchmen, and they get x.p. (albeit at half rate). It can be a pretty quick way to advance a character, provided it survives (which is far from assured, for all the protection the more powerful patrons can provide).
The assumed context of, at the least, "the party" as basically a constant -- usually including all members' survival and success as a normative expectation if not (as plainly recommended in 4e) an entitlement -- radically changes the nature of the game.
A Fighter Lord when fresh might have more than 50 hit points, and a High Priest his peer might be able to "tank up" two such comrades in a day. But suppose the Cleric is otherwise disposed, and Lord Lazy-Not when at half his strength has an opportunity to go on an adventure with a lower-level band? The haul won't be much, but certainly more than he'll get by staying in bed.
High-level spell-casters run into the problem of not having enough hours in a day -- or, eventually, in a week -- to replenish their stocks of spells.
And of course the fellows off on a wilderness adventure that's already a week into the "future" can't teleport back to town in time to join today's expedition into the underworld beneath the ruins of the Cursed Chateau.