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Indeed, this is a problem that comes up in competitive games really often, and the solution to it is often difficult to handle, but given the design philosophy of Ashes I'm not terribly worried. My understanding was that Jeff intended people to adjust strategy based on enemy, so there will probably be things that the Tank/Tank is best at Tanking outright, where the simplistic 'all-in' form works optimally, and then there will be other things where that's not the case.
That fight with It Who Sunders the Land and It Which Rends The Sky (I might have the names wrong) was a pretty good indicator of that sort of thing, I think. I can see multiple ways to tank those, but I feel that a Spellshield of a certain build would have the 'easiest' time of it (think Teleportation Augment on something).
That fight might not be the simplest thing for a Tank/Tank to do, but in terms of actually 'just tanking' and protecting their group, I feel like they would still do the best, whereas the Spellshield would do things 'differently' and be able to do more overall.
It's hard to stop people from going 'well if you're not doing the most DPS while tanking and also protecting everyone perfectly, clearly you're not pulling your weight'. At least with the 'all classes use Mana' decision, the overall 'how good you are' effect may not be so hard to balance.