How doesn't this favor classes that already have a MS like warriors?
Mortal Strike (and similar effects) are applied multiplicatively. That means they don't become any more (or less) effective as Dampening begins to stack.
Here's a quick, if mathy, example: Say you cast a heal that, unmodified, would heal for 100k. If Mortal Strike is active, that heal is reduced by 25%, for a final amount of 75k (and a loss of 25k healing).
Now, say Dampening is at 10%. Your 100k heal would then land for 90k. Mortal Strike would only affect the remaining 90k healing, putting the final amount healed at 67.5k. The heal was still 25% less effective than it would have been if Mortal Strike wasn't active, but was only reduced by 22.5K -- which, you may notice, is 10% less than the previous example, keeping it in line with the effects of Dampening.
TL;DR: Mortal Strike (and similar effects) are not made any more or less desirable by Dampening.