I've never really bought the 'spike damage' argument when it comes to avoidance. If you just stack health you take the same spikes constantly.
I was a healer for all of TBC (MT in Classic and WotLK) and I never had a problem with avoidance. Ever. I would much, much rather have a tank take 40k damage every 6-7 seconds and occassionally 2-3 hits in a row rather than always
taking 40k damage every hit, every time, for the entire fight. The 'spike' argument really only applies to healers with short attention spans that have no clue how tanking works, who wander off to the other side of the room because they think the 'tank is fine' because he's not taking any damage.
Unless avoidance somehow increased
the size of the spikes, there is no issue. Stacking health instead of avoidance just means you're taking massive damage all the time instead of some of the time.