I don't really agree with that in general. It is often pretty useful to upcast spells that increase number of targets like bless and fly. And as has been noted, spells like mass suggestion and conjure animals get significantly stronger. But mainly, the sorcerer has a pretty limited list of spells known. Even if upcasting magic missile isn't an optimum damage dealer, it isn't terrible and it often makes sense to do that rather than waste lot of spells known on basic dpr.Upcasting is generally a weak option overall, except for specific spells.
I believe empower is the only metamagic that ignores the one metamagic per spell rule. That said, an empowered 14d6 fireball isn't altogether that much damage (probably somewhere around 60 damage, after rerolls). That's not poor damage by any means, but given that you're expending a 3rd level spell slot and 7 or more SP to do so, it's not particularly amazing either. It's a moderate boost to nova DPR that can't be sustained for any serious length of time.