Canonically, the only known use of the mushrooms is to power Marisa's grenades, which can either explode or shoot lasers in a specific direction. I was going to make up a number of goofy side effects the mushrooms could cause for people to play with, such as creating fairy rings that teleport anyone who steps into them randomly, or releasing spores that cause specific emotions in anyone who breathes them. You know: stupid RP stuff for teh lulz.

But yes, I assumed they would not grow beyond Locke City or Vegas. (Or at least that they wouldn't have any magical properties beyond Locke City or Vegas.)