That is only true if you are making a weapon for a specific monster with known resists, then you target the weakest resist - e.g. 100% fire weapon for miasma.
But, if you are making a general weapon (not a slayer) then it actually helps a little. Do the math or use the Stratics calculators to see what I mean.
I have done the math - I was a math teaching major before health made me drop out of college as a Senior. (so, I'd had and passed with As & Bs all the math and statistics courses up to Calculus IV). You're reading too much into those numbers.
The Stratics Calculator (And all the calculators like it) are based on the assumptions
1. That CW is not used
2. The weapon has no elemental properties other than imparted by the metal.
which one or both typically aren't true.
When you start running into the creatures that have high resists overall (which happens to be, most of the time, when you start crafting weapons for specific tasks), the benefits of a weapon getting only the metal type virtually evaporate.
The only creatures that you potentially benefit with a Valorite weapon against are ones that have Physical as their highest resist (which I'll admit is the most common), and even then you're tossing 40% of the resist still against their best resist. In many cases, you'd be better off with Verite (which only leaves 30% in Physical Damage).
Consecrate weapon is 100% Damage versus the worst resist.
Armor Ignore is 90% damage versus ZERO resist.
If a creature's resists are all high, all valorite will do is give you 0-5 more points of damage (typically, the quicker the weapon, the lower the benefit), that typically are insignificant when fighting anything that takes ten or less hits to kill.
An extra 5 points an attack is significant in PvP; in PvM, you'll have to do 10-20 attacks with a valorite weapon to where the damage is statistically different (from using a normal weapon (and might be less, based on the creature's resists, instead of more) - and if you're fighting it that long, you've probably lost track of number of hits already. Consider that if something has 540 HP, 6 hits at 100 is no different than 6 hits at 95 (and in a real game situation, that 100 would more likely be 98 or 97). The chance of just lucking into mobs where that tiny bit of damage will make more than 1 attack difference (with that much variability already present in the hit chance rolls), is fairly slim - especially since most fights are so short. If fights are longer, then you're usually back to fighting specific monsters again, and you exempted them from the discussion yourself.
In fact, it's only when elemental damage is used WITH a slayer, that the amount from a 3, 4 or 5 type spread can become significant, and only if they are weighted toward the weak resists (not without a slayer, as you seem to think). The damage changes from elemental damage are applied to the base damage, and the slayer is a DI bonus - elemental damage is a multiplier, and goes thanks to the properties of multiplication have the same effect if done before or after DI is added. That you think that it helps more with non-slayers actually goes to show how much having damage spread out into the best resists as well as the poor ones NERFS the benefit of using a slayer.
BTW...
In my testing, against creatures with all resists over 40%, I do more damage with a single Armor ignore, than with 100% damage versus their worst resist for the duration of CW (or using a 100% weapon), for only slightly higher mana cost. Have high hit mana leech on the weapon, and only trigger AI every other attack, and you can keep the ball rolling indefinitely.
There was an event on Lake Austin back around 2003 or 2004, where we had to fight some awful baddy down near the Exodus stronghold in Ilshenar (luckily, the EM spawned it far enough away from the regular spawn for it not to be an issue, and it was before paragons).
After a half-hour, we hadn't even managed to start the health bar moving past a tiny sliver of red; most of the time the thing was healed back to full. Finally, it dawned on us that when it did show some damage, it was after someone had hit with an AI. It took some doing (and some people running for backup weapons), but once everyone got on the same page and started spamming AIs, we dropped it in 5 minutes, from full health, when 20+ tamers (and their dragons of WW/mare combos) and a like number of warriors (most using EoO and Consecrate, or valorite weapons) hadn't scratched it for 30 minutes.
