Is damage reduction a bad stat to put on a tank?

So I've seen at least one or two videos on the new Galio and some people complain that damage reduction is stupid on a tank. Now while Galio's damage reduction base is probably too high (personally I think it should scale better with his defensive stats, thus making it so full AP Galio is considerably less tanky than full tank) I don't think DR is a bad mechanic for a tank, in fact I think it's great.

Now DR is not a tank only stat, it's a defensive stat. Champions like and both utilize damage reduction in their base skill set (though I actually feel reduced damage on more). It's also noticeable on and just about every resistance item in the game.

Now I don't know the math behind league, but I'm assuming that DR applies before resistances? Or does it basically supercharge your resistances? Either way, the more resistances and durability you have, the more powerful damage reduction is. You know who has a lot of durability and resistances? Tanks (and juggernauts but juggernauts are usually more about sustain). If Riot's goal is to make tanks feel like the tankiest champions in the game (a position that I feel juggernauts take over because of their sustain) then DR seems like a good stat to utilize.

I don't mean as like a tank version of . God knows we don't need any more of that. But probably some major or minor DR abilities in some tank kits. Some not all as not all tanks should really tank the same. Some already kinda don't. While all tanks tank damage through their resistances, they also have different tools to tank with (I'd say from their counterparts but more than one tank has a shield).

Through sustain, we have and . Yes while these two aren't "drain tanks" or heavy sustain champions like or , they are tanks that rely on their sustain as a defensive mechanic.

Shield tanks like and rely around the best times to utilize their shields to tank damage (Granted Nautilus's shield doesn't have much complexity at first. Usually you want the shield and do not care about the AoE part. I don't main him so idk). Sion's shield can be used as either a sword or shield and the decision usually has to be made before using it (hence why first time Sion junglers complain about the shield not being useful for clearing. Tip guys, activate shield and charge your Q close to each other. After Q lands, pop shield, lots of AoE damage at once there). Then we have who would have to stop attacking you to pick up her shield.

Now granted, Courage of the Colossus makes every tank a shield tank in some way. I won't ignore that.

Finally we have DR tanks, coming to us from the new and I guess because of his shield. These type of tanks better benefit off of resistances than others (I think, idk I might be waaaay off) because of how DR works (again, I might be wrong). They also offer clear moments of when their DR is up, which can be a sign to just tell people to not waste your strongest abilities on them.

That's just my thoughts on the subject. I'm sure it's all poorly formulated. What are your thoughts on defensive mechanics?