There certainly should be the ability to swap from 2 hander to 1 hander and shield, or from single weapon to dual weapon. Should also be able to swap between a slash weapon set up to a bash set- up as different damage types are needed for different critters. I always enjoyed games where this was somewhat class based - a fighter build should have the ability to switch to more sets than a mage type for example, based upon skill and familiarity with weaponry. Often this class distinction was further buttressed by feats granting additional weapon set slots.
Something along the lines of fighters - 4 slots, mages 2 slots, thieves/clerics 3 slots (perhaps some additional variety via sub -class). If I recall correctly, many games such as BG1/2, PoE, DA and others made use of similar features.
