There really is little waiting around for a blood rune. Blood with DRM means I have 4-6 runes rolling over Death/Blood constantly. On top of that I don't often use Blood Tap as Blood since I don't have a lot of rune blackouts already. It's off the GCD so it's really easy to use Rune Tap and I pop it the second my health drops and my warning chimes. I'm faster than the healers because there's no delay in seeing a health bar drop.
I average 5-10% overhealing on Rune Tap, and it averages about 20-30% of my total self-healing, where I'm usually doing about 20-40% of my total healing depending on the fight.
I keep saying. The only limiter on the value of Rune Tap is the user. If you use it well, it has loads of value and it is anything but wasted, regardless of content. To someone who never uses it, or never tries, it is easily wasted.