I'd agree if you'd always get a good group, but in my experience, Fates have been quicker than running dungeons until I reached the Vault (and I'm seeing the same on my 3rd class right now), too many dps are just not pulling enough damage in dungeons.

I still run dungeons while doing some Fates on the side though, just for my own sanity's sake.

That being said, I REALLY hope they'll make Fates in Heavensward's areas worth it in the future. I've done quite a few and while it's not optimal xp wise, it's fun to run in these areas and discover new Fates.