As a counterpoint, fights can be built to be challenging without involving Enrage timers. Just look to FFXI, the MMO predecessor to this one. There were PLENTY of challenging fights in that game, and only a bare handful had Enrages (and those few cases, to prevent folks from "zombieing" open-world mobs until folks literally started having real-life health issues).

I've never been fond of Enrage as a component to difficulty. It's a relatively recent addition to video games in general, and games were fun before this started to happen. If you can take what the boss throws at you, that should be enough.