There's honestly zero practical reason in the game to level any desynth above 100. All of the demimateria in the game can be obtained at that point. HW only added 1 single desynth-exclusive item (Stiper Stones from Aithon's Colt) and that item remained 100% useless until SB launched, and then you could just go mining for them instead.\
People leveling desynth to current levels will probably never actually see a net benefit to it compared to the work of the leveling process, unless they make it mandatory for a future relic weapon (many people will rage) or introduce new Field/Battlecraft demimateria.