Macro is only useful if the person doing the pull timer is using one but not everyone in the party has the luxury of it obviously.
Also, with how the screen messages always stay on for a fixed duration, it's easy to miss a 5s intervall message on screen by something else popping up ("x minutes to complete duty, 8/8 members ready" etc) if you're not tunneling on the chat.
And yes, some classes have very precise timings for their prepull stuff that's between the 5s intervals. Can also depend on the fight and how you need to align something with mechanics. I often had to use skills at odd times in the past and still do on some fights. It's easy to miss the exact timing when going be feeling alone.

I don't see the harm in adding it. It would be a welcome QoL feature, especially in PF.