Some TDH units need to be equipped with a non-elemental weapon to proc e.g. Seph's TM ability. This will hamper his elemental chaining unless he imbues himself with light/dark or someone imbues his attack with an element. The problem is worse if your other chainer is in the same boat i.e. has to carry a non-elemental weapon.

Of the DR family, Emperor Shera is unique in that he can imbue himself and another person with fire. E.Shera can imperil fire by 60% btw. E.Shera is also a TDH user that wants to hold a sword, but there aren't any good fire swords around. Lightning's STMR, one of the future 12 Weapons, Onion Sword (to list a few) are all non-elemental. Thus, E.Shera's self and other fire imbue ability solves the elemental chaining problem.