That's a fair question. I think it mostly comes down to the simple fact that an oGCD like Assize, even though it bundles some healing into it (and is already more interesting for that fact that a pure damage spell with similarly no other interactions would be, since timing could potentially then net skipped GCDs of heal) doesn't so provide what feels like an alternative in rhythm to fillers like Dosis. The oGCDs may instead feel only like a different topping on the same unvaried meal.
You'll see that opinion just about everywhere, though, be it r/ffxivdiscussion, r/ffxiv, the YouTube comments, or in-game. There's something about a priority list --especially if situationally varied-- moving down more than a single line of uptime (GCD or oGCD) that makes it feel more significantly like an alternative rather than just "Always hit this button... and maybe also hit whatever's come off CD (that won't get in the way of hitting this button anyways)."
For those with the plugins or low enough ping to handle it despite the game's poor netcode, it'd likely be cool when oGCDs shake up their priority order, too. (On probably the furthest extension of that, a lot of early NIN mains I knew liked that they could let Shadowfang fall off as briefly as possible, without replacing it early, through choosing whether to Raiton or Fuma, or to Suiton into TA in the same GCD gap, where GCD and oGCDs could optionally further fight for uptime and therefore made each other feel more like real choices.)
Quick question:Also aside: It occurs to me that I really hate DoTs - as I've mentioned before - but don't dislike non-DoT spells.
If the icons for your DoTs/HoTs (without needing plugins) tracked the timer of their respective debuffs on your current target (last living target if none selected), would DoTs still annoy you, or would they just feel more like... soft-CDs?