In an attempt to make something productive out of the hyberbole you're trying to use, I do think Leylines should have a lesser effect on other casters standing in the circle. You (BLM) get 15% speed increase, WHM/SCH/SMN get a 5% increase. Casters tend to bunch together so nothing changes in groups and BLM gets some group utility.
While leyline affecting allies would be nice. leyline is too small to have 3-4 players standing in it. its already an aoe magnet