First, upgrading to a higher [W] damage dice you must spend feats. Casters can spend their feats to upgrade their damage as well... a +1 to all X type of damage is roughly the same as going from a 1d6 to a 1d8. And I am sure PHII and the arcane and divine books will have more feats for you.
Second, people using [W] normally are attacking AC, which is always higher, and sometimes much higher than the other defenses.
I am happy to see folks took the time to stop by and point out the many, MANY flaws in the above post. Allow me to point out a couple more.
First, saying that something "costs a feat" begs the question "so what?" If a feat makes a character significantly better, then the feat has done it's job. If feats were a leaner commodity, we might be going somewhere, but as heroic tier feats go, a feat that increases a damage die is as good as it gets, easily trumping crummy little +1's. The real issue here is that there isn't a feat that provides a similarly large benefit to implement users.
(Of course, as Plane Sailing pointed out, the Superior Weapon feat isn't necessary for there to be a discrepancy--it just exacerbates it).
Second, people targeting AC are generally no better or worse off than someone targeting another defense. Fact of the matter is, monsters are built according to a formula that takes a weapon's proficiency bonus into account so it is often a wash. However, Fort, Ref, and Will defense are three of the very few things that a monster's ability scores actually impact. When a monster actually has a good bonus in one of those defenses, it can EASILY match or exceed his AC. Giants, for instance, have gross Fortitude defenses often a point or two over their AC.
A monster's role can be a big factor, because it essentially provides a bonus to AC that takes the place of the ability score modifier that gets added to other defenses. A soldier is usually much easier to target with anything other than AC. OTOH, brutes tend to be easier to hit versus AC than anything else.
