The only reason I see to consider holding back on fey light is if you want your TP-based DPS to not be starved at specific moment in the fight, i.e. on adds spawn or other phases like that where you need everyone to be able to perform at 100%.

I have been asking this in the DPS section to get a deeper insight in the impact on TP usage and possible change of rotations a higher Skill or SpellSpeed can introduce, maybe that is a factor for some players.