This is as close to a "blue plz" post as I'll ever make.

Blizzard needs to come out and tell us already exactly what spell penetration does. It's a stat that has some utility in PvP but by all accounts is completely worthless in PvE. Yet both the mage and warlock T3 sets have it (10 on warlock legs and 23 between the mage bracers and robe), which is a complete waste of points that could be better spent elsewhere, like spirit or more damage.

That and they stick it on items gained in the high-end PvE raid game (Malice Stone Pendant, Gem of Nerubis, Veil of Eclipse, and the unnamed staff off of Kel'Thuzad - provided they haven't changed that item). The first three I can kind of understand, since they're off relatively easy bosses, but off of the last boss of the hardest zone in the game?

All the theorycrafting in the world since they introduced this stat with Ahn'Qiraj hasn't been able to come up with a good use for PvE. So just tell us already what the deal is.

Thats the general trend, its hard to tell if they have any innate resist and all the addons that supposedly tell you boss resists are way off.
Wish they lowered the amount reduced and put back +vulnerability some how. Right now spell penetration has no use but pvp.

Does it help on something like a spellbinder?

Right, that's what I don't get. I understand penetration on AQ gear which is somewhat PvP oriented. I understand it on random nonset items. But when Blizzard min/maxes t3 sets for pure raiding the way they have -- Faith surely does not have +dam on it for a reason -- and then puts penetration on those sets, that tells me that either Blizzard is very, very confused, or we are all missing something about its value in PvE settings. I'm increasingly leaning towards the former, but it's just weird.

As I don't wear it, any idea if it helps on chromatic stuff? Or possibly on red drakes if your fire and whatnot.

Those don't show up as "n type (n resisted)" as far as I know. So it's not resists, it's just a buff they have that reduces damage done on them by certain spell schools.

It obviously helps there, but most mobs fall into two categories:
1) Low enough resist that CoS/CoE negates it entirely.
2) High enough resist that you aren't going to bother at all regardless.

Fireballing a Red Drakonid is never going to be smart, whether you have +40 penetration or +0.

I suppose if there were more things with 100-120 resist to various elements such that you need penetration on top of a Curse, then it'd help. But there aren't, really.

Remember coe also reduces target resistance by 75, so even if mobs do have a certain reasonable amount of resist, its pretty much null and void in a raid setting.

Just to make the 20 spell penetration I have on my current gear hurt me even more, could someone work out how much damage or crit its equivalence is in ilvl?

In my opinion the only way they could make it a worthwhile stat is by letting -resistance items lower the mob's resistance into negative á la old school CoS/CoE.
This would make the items containing -resistance worthy of thier iLvL

Yes, yes it is. I wish they would put in the mid level resist mobs (100-150) so these stats did something for our PvE gear. My Doomcaller is excellent raiding stuff, but wtb crit instead of -resist please!

If -resists does anything, it does it at a better value than hit/crit%. 1% crit is about equal to -10 resists. At level 60, -10 resists is worth an extra 2.5% of damage, if something is being removed.

At this point, I have to assume that Blizzard noticed partial resists on level 61-63 mobs, and assumed that -resists would remove that. Which it doesn't, since that's a skill based resists that can't be overcome. :(