Sure, you can look at a Tier 1 class in 3e or 5e and say "hey, there's no one thing they do with that much MOAR Power than the Tier 2 class that also does it well." A Cleric abusing Spirit Guardians for instance, is no more OP than any … other...er... it's not actually on anyone else's list, is it? nevermind, bad example...
...but the point is, if you can swap out all your spells on a daily basis, on top of dynamically choosing which of them to cast on a case-by-case basis, you're a LOT more versatile than Either a caster who picks those spells at chargen & level up, OR a 3.5 Tier 1 caster who prepped into slots, old-school-Vancian style.