I do think side quests could stand a bit of an XP buff. It's possible to use them effectively for leveling, but to do so you really need to stack the buffs. Doing the MSQ on GNB (to duo everything with my in-game best friend who mains WHM) and leveling WHM alongside, I was able to use the side quests (plus a handful of FATEs) to effectively keep WHM almost on pace with GNB (generally about 80% of a level behind). But I had the armory bonus (any time WHM didn't catch up, anyway), plus I'd made a stockpile of Grade 3 company XP buffs ahead of time and the FC kept that up for the first month of Endwalker, plus I always had the +3% XP food buff going.

I recognize side-quests are more meant to be things that flesh out the world for lore-nerds like me -- and I will continue to do them all for that reason, regardless of XP -- but it does feel like it'd be nice if you didn't have to stack quite so many buffs in order to make them also effective for leveling. I think it's fine if they don't give you as much XP in a zone as the MSQ does, but it'd be nice if they gave a little more than they presently do.