I think they should keep the d12 for a two reasons.
- It is perfect for HD for truly beefy creatures and classes
- It is the ideal die for melee attacks that do a great deal of damage.
- It has an intimidation factor that the d10 does not have when used for damage.
- It is more convenient to use than 2d6 (no adding)
- It generates number from 1-12 without a curve (as from 2d6)
Some explanation:
The d6 gets alot of appreciation, but that is only because they are the easiest die to get alot of. The fact that you can get them with every other board game ever invented is why the "Roll X dice per level" spells use them. For spells requiring gobs of dice, I would go with the d6 or d8 anyway.
Other than being easy to obtain alot of for casual gamers, the only other thing the d6 is good for are weapons that non melee types have to use, low HD classes, and rolling base stats. Alot of people like to use point buy now anyway, so that last one does not carry as much weight. But for the foreseeable future, the d6 is king when you need Gobs of Dice.
But Gobs of Dice are a pain in the ass to use frequently. Rolling an 8d6 fireball does create a cool moment, but it can bog things down to add up.
Now, if you want to do alot of damage but only roll 3 to 5 dice to do it, your best options are the d10, the d12, and the d20. The d20 is not ideal for a damage die unless your only rolling 3 or less. While it is still trivial addtion, you are having the possibility of adding up say, 19, 17, and 15, and 8. 59 damage on 4 dice is not bad, but it is not the fastest set of numbers to add up. And the range is a bit too wide for using many dice. That leaves the d10 and the d12.
The d10 also fits for doing beefy damage and solid hit points, but to me, using something based on d12's is like "Turning it up to 11". The d12 does not get used for many things, but it is a big die. It is quite frankly more intimidating to the players to start collecting a handful of d12's for something than it is to grab d10's, despite the average damage range not being that much higher.
Also, consider this. 4th Edition will have 25th level spells and 30 character levels. Having spells that do half your level total in d12's for damage gives you half as much dice to add up as using a d6. It offers a comparable maximum, but also gives you more variance in the total. And more variance, at least to me, is a good thing.
END COMMUNICATION