Any macro including an on-GCD skill has the potential to induce delay. Say you cast Fire, 2.5s GCD. You press Fire again when you still have 0.5s left, you'll begin casting Fire as soon as the first is finished. If Fire were part of a macro, if you pressed it at 0.5s left then nothing would happen. You would have to press it after the previous GCD completely ended in order to begin casting it, and unless you can somehow perfectly press it at the exact moment every time, you're adding small delays with each cast when using a macro.
Can just press it once; it will persist even through multiple casts until you move, change targets, or press it again.
