If EH is king and you can neglect avoidance completely, we should all ditch our tanks for Sarth-specced Warlocks to have their VW tank & have a raid full of rogues & Hunters MD threat on it. But the fact you're still being invited to raids (other than to pick up the tanking loot drops) is because avoidance DOES matter.
I'ld even state what has been stated back in tBC:
If you have 1 more HP than the worst possible combo of damage a boss can do to you before getting healed back to full, you've maxed out your EH, as all the rest is wasted due to not being used.
Oh, and (speaking as a healer) you're wrong in assuming that avoidance doesn't affect healing received. Absorbs (done by both disc priests as well as a little from holydins) are usually consumed faster than they can be applied. By neglecting avoidance, you increase the chance that the swing previous to a worst-case spike burst scenario will have consumed said absorb. The same goes for Earth shield. Therefor, avoidance more hits will have absorbs/ES up more when they're needed, namely the big OMGHERIPPEDMYARMOFFHEALPL0X!!1 moments. You're not the Black Knight after all :P