I can see both points of view and have mushroom identification questions of my own that I've been mulling over (every time I see a new kind pop up in the backyard in fact).
First I thought...
Given that you're not so much identifying the species but whether they're edible or not (or how edible) then perhaps such questions genuinely belong on the Cookery SE, even though the members here are probably more likely to be able to answer.
As an aside, check out the bottom answer on this question: https://cooking.stackexchange.com/questions/3225/how-can-i-tell-if-a-mushroom-is-poisonous
Then I thought...
If you want a positive species identification then arguably the newly minted Biology SE could even be a valid target for such questions.
Then I concluded...
Not appropriate anywhere?
Even so, regardless of which SE should be the target for the question, maybe a generic Stack Exchange (like Gardening, Cookery, Biology) is simply not the right way to go about identifying a mushroom for consumption and so should actively be discouraged across all 3 SEs.
If a "mushroom identification" SE ever gets off the ground I'll be in there for sure.
New Area 51 proposal: http://area51.stackexchange.com/proposals/41570/mushroom-identification
Actually, I had a crack at it -- anyone wish to hop on board or help me with a better description/title?