Mushroom Turrets are useful for some extra bonus when you really need it. It produces mushrooms near the base, which temporarily boosts the stats of the heroes depending on how much it grew. Unlike in the Garden level, every single mushroom is red/green and increases both damage and speed when eaten. More than one of these is not needed and is down-right stupid as the mushrooms don't overlap the bonus.
Coupled with some turrets that specialize in killing grubs, these turrets will be very useful, as the mushrooms can make Spider, Ladybug or Stickbug easily kill the bigger enemies or let the faster heroes explore extremely quickly. The best part is because of being able to grow mushrooms in a considerable amount of time; upgrading these turrets can be left for last, unless you seem to be in big fights all the time.
Additional Note: Oxynidus, the biggest pro of Bug Heroes, loves these turrets!