Hello Lynea!
Whilst you may disagree with my points, there are elaborate explanations that I did not make regarding that.
First of all, I based off my points for a DK that is mainly going to tank Lich King - which means that my points suggest about tanking the encounter more rather than any other. Whilst I may be wrong about hit rating (I may be wrong, but I heard some things were pretty buggy atleast in DW), glyph of disease IS a preference. My glyphs are: command, vampiric blood and disease. Here's why:
I used to be in a guild that had always made DKs off tanks, no matter what. They are more likely preferred to off tank, especially if they know what they are doing, and we lacked tanks that knew what they're doing. As an off-tank, trying this glyph was not a disappointment as it increased my range I can put diseases on ghouls that are near the main tank, and therefore gave me bigger chances of being able to pull them towards me with Icy Touch, as it gave me more threat no matter how much cleaves, starfalls and divine storms are all over the place. Whilst you may think this is completely useless, but atleast for a geared tank this could be a preference and not mandatory, more like the third choice you can make.
Regardless with AoE, don't bother! Don't worry if you can't hold multiple targets at once, but definitely don't pull whole ICC unless you have a hunter/rogue, because you are more likely to kill everyone. Otherwise, spreading your diseases, DnD and ocassional BB (as long as you have your diseases up) is the way to go.
Regarding Heart Strike, I don't think it gives threat as much as Icy Touch and your other main abilities that I included, which doesn't make it that useful, also with the point you made in your detailed post. Many tanks are able to tank without it as it doesn't provide you much - whilst it may do multiple target damage, so does DnD and BB, and on single target, trust me you would rather stick to IT+RS and save your blood runes. I assume it would just depend on how well geared you are.

