Is it possible that the spores summoned from a mushroom circle still decay even when placed in a shop stockroom? I think it is bypassing the original limitations of the skill by allowing them to be kept indefinitely until desired for use.

That's rather like saying I think great robes or armour or anything else that decays, should continue to decay in a stockroom because it is bypassing the original limitations of the skills that made them.

Not exactly, spores are a means of instant transportation, if you can stop spores from decaying then that means if an enemy gets hold of spores leading into your territory, you'll never know when they strike. Whereas, if they decay, you have at least limited safety and control. Or not.

*cough* to be more in accordance of what Elryn said: Spores decaying is a built-in limitation of their potential, which is important for powerful skills like spores. Spores are stopped by very few things, so a short decay time is a way of preventing people from having instant, barely stoppable means of getting into enemy territory at their disposal any time they want to. I agree with Elryn, it'd be good to have them decay no matter what.

Scrying is a way of instantly locating your opponent, should we have scry enchantments decay? Armour is a way of instantly reducing damage against you. Everything has a purpose. Safeguard your spores instead of planting a circle just cause you have the awesome cool skill that lets you do it.

I still think it's not the same but for the sake of getting people to finally stop summoning mushrooms wherever they want (though I feel at least the Glomdoritos finally got that reigned in pretty much) I'll just agree with Gregori and say maturity's always better than calling for yet another game mechanic (not directed to Elryn, of course)