You might get more detailed responses if you posted this on the fungi forum but basically picking a mushroom/toadstool is the same as picking an apple from a tree. In order to make a correct identification it is usually necessary to pick one specimen to ascertain whether it has pores or gills, to photograph the underside to show such things as the gills, the way they are connected to the stipe, shape, colour etc. Some will also take the specimen home to make a spore print and for microscopic analysis which in many cases is the only way to determine the identity of the fungus.
Having said that, I do not see the point of picking more than at most a few (young, mature etc.) to photograph as a collection to illustrate the different appearance of the fruiting body at various stages in its lifecycle.
I'm not certain, but I believe many will still release their spores (which is their sole reason for being) as they dry out and rot away. Most of the life of a fungus carries on unseen underground (or beneath the bark of a tree).
Sadly, I think far more than are picked by photographers are kicked over by people with the "Ooh, a toadstool = bad!!" mentality.
