Based on what I've seen in other games, I do believe you are right. I hope not, but still...
Ash Cloud is a good contender for the Obsidian Shield slot, but as irons says, it will probably not trigger the +4% stamina passive 
Although I could probably do without that stamina boost. I'll have to do some more thinking 
The thing with Ash cloud is that you have 30% miss chance which means if it runs bad for you get the full dmg of the big hits in.
This isn't really what I was trying to argue, I was saying that the diminishing returns on % dmg reduction makes it not worth taking spiked armor, obsidian shield, immovable, and whatever else % reduction there is at the same time...
And, yes...I don't know if that's how it really works, but if it works additionally rather than being multiplicative then the game would be broken, so I actually would bet you money that I'm right.
I wasn't talking about regeneration at all, just the Dragon Blood heal + having the regen active for the 12% incrs healing if you take Obsidian instead of Spiked to get that passive... as long as you have the +12% incrs healing, the +28% regen+7%+5% is just decent..this'll probably be additive because it's beneficial positive rather than negative benefits, but the difference here between additive and multiplicative isn't much anyway. Let's pretend it's additive and do the math. If you have Dragon Blood on all the time and that's your only +5% ability then you're getting +40% hp regen, which is only 1.2% extra per second on a base of 3%. That's a big difference over time but if you're getting bursted it's probably nothing worth jizzing over. I wouldn't worry about getting that +% regen...but this really is just heresay at this point, since we don't know how many seconds the fights will last it's impossible to calculate the actual difference that % per second will make.
Spiked VS Magma depends... but my point is that because of DR you should just take one or the other, there's so much DR (because I really do guarantee it will be multiplicative) that any other form of ability will be a better choice.
It has to be right, there's no way that 40% + 30% + 20% reduction is going to apply like that. 100% x .4 = 60%, x.3 = 42%, x.2 = 33.6% damage taken rather than 10%. It's still really good, for sure, but it's not as good as other distributions because on top of it being multiplicative there's also going to be soft-caps if not even hard-caps within every stat...so you'll want to divvy it up as even as possible.
First of all you shouldn't compare Spiked Armor with Magma Armor.
Better compare Spiked Armor with Obsidian Shield.
Spiked Armor gives you a reduction of 30% a 5% passiv reg + 12% heal while active
While Obsidian Shield has a reduction of 40% costs less is an AoE and gives you 5% stamina back
You take Dragon Blood for the strong self heal of 25% you wouldn't try to spam it for the reg bonus.
It has to be right, there's no way that 40% + 30% + 20% reduction is going to apply like that. 100% x .4 = 60%, x.3 = 42%, x.2 = 33.6% damage taken rather than 10%. It's still really good, for sure, but it's not as good as other distributions because on top of it being multiplicative there's also going to be soft-caps if not even hard-caps within every stat...so you'll want to divvy it up as even as possible.
Edit: Just take obsidian shield instead of Spiked Armor and you'll still get Helping Hands, yeah? 
When you do that math don't forget that the first dmg reduction is 4% and comes from heavy armor.
I would never use 3 dmg reduction skills unless it's neccessary because I think it would be overkill.
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A different thing:
Tanks as DPS or How much dmg will a Tank be able to deal compared to a pure DPS?
If we take a look at all the dmg reduction a Tank has compared to normal DPS.
There would be more Armor, more HP, higher HP reg more incomming healing, dmg reduction, better blocking.
And if you take a look at the skills there aren't many heavy DPS skills but lot's of utility or defensive skills.
So do you guys think that a Tank will be able to keep up with a pure DPS?