As someone who currently mains a healer, and has mained a healer for a very large portion of my WoW career, I can tell you that the only two types of healer in WotLK that are going to tell a tank they are easier to heal are 1) bad healers, or 2) a holy paladin that is spamming Holy Light and only makes the claim because their holy light casts are useless on you more often than they otherwise would be.
A tank build for avoidance, by nature, has less EHP (this includes armor and overall mitigation) than a tank build specifically for EHP. This means that even though the avoidance tank takes damage less often, when they do take damage, they take more of it. If I'm a nuke-heal-spamming Holydin, it doesn't affect me *that much*. If I'm literally *any* other healer, it's going to make a huge difference. Because that is forcing me, a holy priest, to react with more panic, rather than just casually and occasionally pinging you with heals.
There are only two situations where you should actively seek out more avoidance.
1. When a boss hitting you two times back-to-back will most likely kill you. Such can be evidenced on fights like Brutallus back in TBC, where otherwise EHP-built tanks would instead use an avoidance set. If getting more EHP will not increase the number of hits it will take a specific boss to kill you, then it is OK to get more avoidance instead of more EHP. But unless you're willing to get down to an exact science and switch gear out on a per-boss basis, then you're not going to benefit from this.
2. You're playing an Unholy DK where the up-time on your Bone Shield benefits from avoidance, and thus making the effect last longer. But just the same as the above point, this is only useful until a point.