If you have two skills, one a 15s DOT that is on the GCD (50p per tick, total 250), and one a 15s CD GCD that is 250p, the function is pretty similar in a training dummy scenario. They're both 'use once every 15s for 250p. But the DOT comes with additional nuances. You can use it on multiple enemies at once with multidotting (assuming your AOE skill doesn't dwarf it at certain target counts), you can purposely clip it if you need to for...whatever reason (eg if it has on-cast damage like Dia and you're needing to move), and you can make the call to not refresh it, if it's duration won't fully tick. As an example, in P12S doorboss, there's a point just before limit cut where the DOT falls off. I don't know if it's worth refreshing it there or not because the boss jumps before it resolves, but I've not looked close enough to see how many ticks I'd get out of it (and whether that beats another Glare). Dia being a standard 30s CD GCD ala Goring/Sonic would not have that option, it'd just be 715 potency burst and the question answers itself instantly
I think because of this, the issue to solve, that of 'DOTs are annoying to manage', should not have been addressed with 'convert some to burst hits, and consolidate others so there's only one DOT per class left'. Rather, improving the UI that allows you to track DOTs, would probably have been better. Like if you could see 'only your DOTs' on the enemy frames, emnity list, whatever it's called. Hang on, it's Paint time again:
Being as we're mostly asking for 3 DOTs as a maximum for healer redesigns (mostly), this means that the number of icons shown on the list would be, at maximum, those 3 DOT icons. Other player debuffs (Trick, Mug, ally DOTs) or your own effects that are not DOTs (Chain Strat for example) would also not be shown
oh edit: in case it's not clear, this is two examples in one picture. If you're a SCH, the WHM Dia would not be visible to you. Instead you'd see the Bio/Miasma/SF icons (if they're present) on both targets, similarly as a WHM you wouldn't see the SCH stuff, just whether or not each enemy has Dia on them.