I've always viewed Crit vs. MP5 as the difference between Avoidence versus Mitigation for tanking. Sure, crit will refund mana in big chunks just like parry or dodge will avoid 100% of an attack. However you cannot count on it. MP5 like Stamina or Armor is always on, dependable regen.
This is not quite right. The Avoidance versus Mitigation debate has had the side-effect of promoting some incorrect beliefs about probability. The reality is that:
You cannot count on probability in the short term.
You can count on probability in the long term.
The reason you can count on the long term is something called the Law of Large Numbers. It's the same principle that casinos and insurance rely on. The key here is determining what is long term, and what is short term.
Tanking heavy-hitting bosses is inherently short-term. For some bosses, as few as three or four hits in a row will kill the tank. When the number of events is that low, you cannot count on probability to keep you alive, and probabilistic or Avoidance stats like dodge rating are less useful than stamina or armor.
However, if it takes a larger number of hits to kill you, probability becomes more and more reliable. For example, I tank the murloc adds on Tidewalker. Each add doesn't hit very hard, but there are many of them. It would probably take 10-15 full hits to kill me. In this situation, Avoidance stats are actually very useful.
For mana regeneration on healing crits, you are considering a large number of spells over a 10 minute fight or so. That's upwards of 100 spells cast, which is definitely a long term issue. So you can count on Illumination to return a reasonable amount of mana.
On the other hand, because it only takes one or two spells to heal the tank up to full again, you cannot count on spell crit to increase the power of your heals. Yes, over the course of the entire fight your average heal will be higher, but relying on the next heal to crit is a bad idea.
See the difference? Mana regen over an entire fight is a long term issue, but returning a tank to full health after a big hit is a short term issue. Spell crit helps with the first part, but should not be counted on for the second part.