@Maker
If you have a 0 Follow Through Time, and if you have a unit that has Spell Animation (which most units have it), the spell would be activated before the unit finishes its Spell Animation, that is why it is recommended to leave it around 0.50 ~ 1.00 to suit any unit's Spell Animation.
If you look at Rexxar The Beastmaster, he has a Spell Animation that looks like he's shouting in front.
If you set it to 0, the spell would be triggered before Rexxar finishes its Shout-like Animation, visual-error.
EDIT:
It's not about the spell, it's about the Unit's Spell Animation.
If you set it to 0, it would end the Unit's Spell Animation instantly, making your unit to be somewhat, retarded.