Rng mechanics could probably do that, but at that point how effective would a meter even be, if the boss has enough rng on it to create completely new encounters each and every time.
In MY experience this is a misunderstanding of what Trackers are for.
Trackers in FFXI do two things.
1. Help you confirm that your build/gearset is achieving the goal you built it for.
2. Make sure everyone is on the same page without having to save and scroll through every combat log (game provided) for long battles.
While an FFXI boss is not exactly 'a new encounter every time', because just as Steven says, you need to be able to plan your builds, and your team composition,
beyond
that you can get some wild streaks. I believe I've mentioned it before but some days Jormungand acts like a Mage, constantly using AoE and blasting out paralysis, and some days it acts like a brawler Warrior, attacking an insane amount, building lots of TP, and shredding the Alliance with TP attacks such as Spike Flail and the hate reset Horrid Roar.
People literally argued about the best way to defeat it because group A had a long streak of 'Mage Jormy' and groups B and C mostly met 'Warrior Jormy'.
So the Tracker in that case would be telling you things like 'If I build to counter the Dragon by adding more Ice Resistance, and I think that I will still do enough DPS because I don't think it is THAT evasive, am I correct?'
And similarly 'If I build for more DPS and accuracy to make sure that I keep hitting it, will I still fail because the rate of the Paralysis Proc is too high for me to use all that Accuracy and Damage?'
We're talking about a fight that takes 30 minutes, in which some characters perform 900+ melee attacks. A lot of the more 'casual' Tracker users that I'm aware of, this is what it is for.
You can't test a change in the accuracy of your build 'on Jormungand' by fighting a dummy/scarecrow. You can't work out 'how often out of my 900 attacks was I paralyzed' without spending more time number crunching than you do playing the game unless you have a certain specific weird 'skill'.
So I guess people could theoretically just 'guess and hope', particularly because they could be attacked mid combat anyway. But if builds are meaningful, and the game has Acc and Eva aspects that matter in this way, the amount of frustration from 'not parsing' will often change the behaviour of many players.
RNG build stats -> Tracker usage. There's a clear benefit, almost always.