The main uses I can imagine are:
- Fights where adds spawn at predictable times and places. You can put mushrooms up at the spawn location. However, I haven't managed to do this even once. It's hard to get the mushrooms placed in the right place.
- At the beginning of a fight. If you can get the tank to tank the enemy over your mushrooms, you can get a little dps boost. Emphasis on little, however, and tanks right now have enough else on their minds.
- A slow zone for kiting. There's a talent that makes your mushrooms leave a slow-zone behind, similar to one of the hunter traps. This might be the best use, though again it takes some coordination. It could be useful, for example, on the last boss in Blackrock Caverns.
- Targetting them takes a few seconds. You can't just work them into your rotation, because you have to cast the spell and then place the targeting reticule.
- They have a tiny, tiny area of effect. You have to get them in exactly the right place.
- Triggering them takes an extra GCD. So you not only lose time placing them, you lose time when you are ready to blow them up.
- There are three of them. You can't just drop a shroom and blow it up. You want to drop three shrooms so as to get the full effect.
Let us compare the mushrooms to a hunter trap. Both can be placed at a distance, but while the hunter trap detonates when it is hit by an enemy, Wild Mushroom must be triggered in the one second it takes for a monster or character to run over it. If a group of mob triggers a hunter trap, they will all be hit by it, but if a Wild Mushroom is triggered near a group of mobs, only those very close to the mushroom will be affected by the damage.
Personally, I'm not sure how to make them work well as a damage ability. They're either strong enough to use them on cooldown, or they're not, and if they ever got that strong, it would warp the game balance in other ways. For example, now Moonkin would have an inordinately powerful trap to use in PVP. I would rather other areas be focussed on than raw damage. For example, maybe the damage could be taken out, and the debuffs emphasized. They could last longer, they could slow without any modification, and if you get a talent and/or a glyph they could apply some other debuff such as a slowed attack speed. A debuffing mushroom I think I could make more use of than yet another way to cause damage.