For the general non-hard mode tanking population, glyph of rune tap is a bad glyph.
A few readers noticed (and one commented on other forums, in fact in their guild application as a DK tank) that in my grid of Blood specs, the hard-mode specialised AOE spec includes this glyph.
Firstly, how’s its maths work?
Your Rune Tap heals yourself for an additional 10% of the effect, and also heals your party for 10% of their maximum health.
This means, when you have 4 points spec’d into [spell]Rune Tap[/spell] for its 20% of health self-heal, you gain 10% of that, making it 22%. You gain a personal heal of 2%, which is +1200 health for a 60K hp tank when compared to an unglyphed rune tap.
It then heals your party for 10% of their health. I don’t know its range but expect its 20 to 30 yards.
So when is this glyph useful?
Most of the time it is not.
PvPers might use it for the party-heal. It’s +2% self-heal is quite meagre too.
This glyph is useful when:
(a) your party is not at max range and are taking AOE damage, and
(b) you need all the self-healing you can get, and
(c) single-target or AOE threat is less important than survival, so your glyphs can follow.
The only fights I can think of where this is the case is the off-tank for Lich King hard-mode (which is why it’s in that specialised spec) and any tank on Sindragosa hard. The party heal helps with Infest and the slightly larger self-heal has a (small) probability to be useful at a few moments in the fight. As you probably know, I haven’t personally tried those fights on hard-mode; so here I am combining theory with the discussion in forums.
The same caveat for rune tap also applies to this glyph: it’s only as good as the tank who uses it and how quick their reactions are.
Comment on this glyph here on the blog on in this thread in forums.