It sounds roughly like a Bromeliad (a type of flower which holds symbiotic pools within itself). It sounds like little cost to the tree (it could just collect rainfall, or pump up ammonia from the ground into these pools), but it'll give an opportunity for a predator to move into the area.
However, there are cases on earth where plants develop fancy defenses, but insects sabotage these defenses, allowing them to safely predate upon the plants.
Overall, I won't say if one group will succeed, or if the other will succeed, but at its core the symbiotic relationship is plausible.