As someone who has alternated between tanking and healing in high-end raids, I can say that avoidance is never a negative thing. It can only be a positive thing. If you want to just stand there spamming heals, you always have that option. They won't do anything, but it won't hurt either.
However, tank avoidance gives healers the ability to choose their actions more freely: toss off a GCD on someone else, move out of fire, recover a touch of mana, refresh HoTs, etc. Avoidance buys time and time buys flexibility.
Even if they do not gear for avoidance at all, it's a good idea for tanks to understand the impact of avoidance on their healers. So often I see the fallacy that avoidance is a negative impact on healability, and it simply has no merit. If you are healable without the avoidance then you are at least as healable with the avoidance and most likely more healable--in addition to being more likely to allow your healers the ability to do other things which will benefit the raid.
I good practical example for this that always comes to mind for me was back in Black Temple when I was always one of the primary healers on the Gaithos tank in the Illidari Council. Gaithos could hit like a truck but hit slowly as well. When the tank took damage he took shittons of damage, when he didn't you knew you had 2s of breathing room. Avoidance on the tank meant that: a) I could move out of bad stuff without the tank dying, b) I could throw a Flash Heal or Renew on someone who was low on health and at risk of dying to bad stuff, c) I could regen some mana or use a mana cooldown/mana regen tricks without risking the tank on a very mana-intensive fight. At no point was I saying to myself, "I wish the tank got hit by every swing so his damage was more predictable!"
So, as Satorri says, the healing element is where things get complex, mostly because healing is a limited, triage-based resource in addition to healers typically having to be more concerned with interrupting what they were trying to do for various reasons. This is not something that is easy (or even possible) to model.