What an interesting idea, I can't help but visualize Zangermarsh[1] :-) Interesting question about what it might gain evolutionarily by being tall, perhaps more moisture exposure but as others have pointed out fungal mats get that by just being wide and covering a lot of area. Of course if you were competing with a fungal mat being tall might be one strategy.
And by taller than others the spores would be more likely to be carried over everything else by the wind and travel further distances. A short mushroom's spores might end up only traveling a few feet before hitting, say, another mushroom.
Haha that zone totally came to my mind too. It's actually quite cool how many different types of zones Blizzard has made in that game... I wonder if they'll ever run out of ecosystems.
[1] Its a World of Warcraft zone