If Cursed Wombat leaves the battlefield while one or more triggered abilities generated by the ability it grants are still on the stack, those abilities will still resolve as normal and put additional +1/+1 counters on the appropriate permanents
With planning, a careful reading of the rules, and a focus on strong underlying games, these promotional rounds can add variety, structure, and value to your regular sessions
Not only does it grant them an additional +1/+1 counter, it also reduces their cost by up to two, and allows you to draw cards off the back of casting them
The flipside of giving legendaries the power to escape turns another creature into a commander of sorts, and a decent way to bypass commander tax if you can fuel your graveyard
I would only play this in the 99 of a Golgari or Orzhov life gain deck