Because FCR6 is comparable to FC2 property wise.
FCR6 actually saves more time in your overall casting, especially if your juggling curse weapon, consecrate, divine fury and EOO
This couples nicely with protection because fcr is not effected by it.
Protection lets you guarantee your cast as long as your skill point are sufficient.
This is great against multiple mobs, bosses that cast wildfire or use insanely strong casting debuffs.
The casting debuff is especially gruesome when not in protection.
Try fc2 and try fcr6, cast several buffs in repetition and you will notice that cast recovery being 0 makes things very sluggish.
Most of those spells are quite fast already though, even at 0 FC they are reasonably fast.