This falls under the "way too hard" syndrome.
In the lifetime of this community (praise be the Eric and praise be the Morrus) we have run across numerous places where certain rules fall between the cracks, just by virtue of human omission. Just because they didn't say "a helpless creature doesn't get a reflex save" yet have numerous other places which imply that if you can't move, you can't defend, people will defend to the death a completely implausible rules condition.
It's at that point you have to state your argument, agree to disagree, and move on.
If you wish to give immobile people reflex saves, by all means go ahead. But in my opinion it goes beyond the bounds of applying rules logically with common sense.
BTW, as for stunned creatures - as Caliban said, stunned is not helpless. In fact, stunned creatures still get the basic 10 points of Armor Class that the dirt-basic 1st lvl commoner gets! This is best represented by the person who has been hurt badly, and is not into taking actions cognitively, but still knows that something bad is happening, and they "have to get out of the way."