I am confused.

Literally the only thing the macro does is make you face target for fire ball. Exactly like pressing F while casting. It does not slow down the cast or make it cast later than normal. Use the macro for controller, it helps in the exact same way pressing F does. For FCoB theirs lots of movement the bosses do, so being able to turn mid cast helps a lot. It doesn't interrupt you. No negative impact, no reason to not do it.

... IMO. But to each his/her own!