As was mentioned, GLD/PLD used to be able to use daggers. Once ROG/NIN was introduced, SE was forced to turn all those daggers into swords, as ROG/NIN was now the class/job identified with daggers.

If SE was to give weapon variety now, they're setting themselves up for future issues of exactly the same type. Suppose they give GLD/PLD the ability to use maces, and then later on decide that they want to add a new Templar job that uses maces as their primary weapon? SE would once again have to go back, change all the GLD/PLD weapons into swords again, so there's no weapon overlap.

The multiple weapons per job thing was a remnant carried over from 1.0, and one that's caused SE problems while developing for ARR. I seriously doubt they're going to go back to that mess, even if some players would find it to be appealing!