Actually, allowing a Lancer to be a White Mage will probably prevent the player from using any lancer abilities.
The blueprint did say that certain jobs will prevent equipping of certain cross-class skills. Most Lancer skills will likely be in this list for WHM.
So, who wants to play a WHM wielding a barely-effective spear and possessing a reduced MP pool? Because THAT is what the OP is asking for. It's probably not what he wants, but it's what he's asking for.