Gez said:
Notably, against trolls, mummies, frost worms and white dragons.
Sure, and against things like red dragons, fire giants, hellhounds, efreeti, and salamanders, it's absolutely useless.
When it comes to the Protection From Elements type of spells, yes, all elements are equal. But that doesn't mean that OVERALL they're equal.
Fire and Cold are the only two damage types that come naturally from "environmental" effects. I mean sure, you could fall in an acid pit somewhere, but just living normally in certain areas? Definitely Fire and Cold, and maybe a little Lightning.
So, there are a lot of Fire and Cold subtype creatures out there, immune to their own element but who take double damage from the opposing. So far so good, right?
Then, look at what spells are the most common. Sure, if you add in every splatbook out there it dilutes out, but just from the PHB? Very little Acid or Sonic, plenty of the others. So, when people cast Protection from Elements as a pre-combat preventative measure, which elements do they pick? (Hint: not Acid)
I'm not even going to get into the "Celestial, Fiendish, Axiomatic, Anarchic are all resistant to Cold and 3 of the 4 are resistant to Fire and Electricity but only 2 of the 4 are resistant to Acid and Sonic" type of thing, because nitpicking about resists of individual creatures/templates gets us nowhere. Other than subtype creatures, we could assume all resists are equal and it'd STILL make, say, Acid better than Fire.
The question is, which is more useful for a weapon ability: 1d6 Acid that works basically all the time, or 1d6 Fire that doesn't work at all 10% of the time and does double damage 10% of the time? Using the "randomness is bad" principle, I'd go with the first one.
Don't believe me? Put a Wizard with Energy Substitution into your campaign and watch what he does. A friend played one in a campaign I DM'd. Practically every single damage spell got substituted to Acid or Sonic, unless he knew for sure he was facing a subtype creature, and even then he'd still leave most of them Acid/Sonic just in case.
So yes, I'd say there's a heirarchy of elements. If you want to change Thundering to be more like the other ones, trade the Fort Save deafen for 1d4 sonic damage, maybe. But, if you make it 1d6, why would people take the other Burst abilities?