I don't see how it isn't lame, knowing it cost absolutely nothing to SE. Being able to play each jobs in the game is not a valid reason. As said above, you may want to play on different World. And we aren't 100% sure yet that everyone will be able to level all crafting classes to maximum level in the future.
When PvP will be here, some players may want to play in different factions/company or whatever it will be. That can only be achieved through one mean, having an extra character.
Because it's a part of their business model. There is also a reason why SE tried to "improve" the retainer system rather than moving on to a straight and simple AH. Retainers ? Have to pay for them. AH ? Doesn't generate any extra income (unless they start doing like Blizzard with premium smartphone apps).why not just charge $15 and allow players to make a bunch of characters/retainers, can give it a cap at like 50?
To me it was clear since day one that whoever made the retainer system took the $1/retainer business model in consideration.
(definately not the only reason, but still... money always matters when such decision are taken).