@Werescythe, in Sims 3 plantsims were better gardeners than other Sims, could send flower kisses to other Sims and could also poison them with poison kisses. They also could absorb puddles to replenish their water motive, which was a cool way to clean up puddles.
Beyond that, It would be cool if they took another page from DC Comics' Poison Ivy and gave them the ability to make plants grow, and it would be even cooler if they added "mega-sized" varieties to all the plants that only a the Plantsim's growth ability could achieve.
There's also the mind controlling abilities, but at that point they are just rehashing things we already have in the game through other methods/lifestates.
Ultimately, you aren't wrong that it is not a particularly interesting lifestate, but it could at least be a LOT more interesting than it is right now.