Mainline diameter also matters when it comes to balance of your rig. Using a .12 mainline and say number 2 elastic would be daft, the mainline would be too powerful really for the elastic.
Also, finer mainline gives better presentation in windier conditions, Here in the fens that makes a whole load of difference. I tend to fish .10 mainline for most silver rigs, commercials, heavier river floats and bagging/positive rigs 0.12 main for durability, and .08 for winter/canals/fishing fine for small fish. Works for me.