I'd think it was mainly down to shields being made 0 slot. The slot system was brought in to stop characters being omni-competent, back when everyone was carrying a 2H for normal combat and a pike to skewer cav, and a crossbow & bolts as a sidearm. Taking away the slot for a shield encourages all shielders to put a few points into PT/PD or a some wpf into crossbow so they can have a sidearm again.