While the ecosystem has been severely impacted by the transfer of plants and animals to places that they would not have normally been able to get to by humans, and the whole planet is on the downhill anyhow with unatural polution and such. It would still be a good idea to try to keep these "incidents" to a minimum. You never know how much of an impact it may have. The new population may survive and flurish for 5 years, then grow to big, eat all the available food, and starve to death, the the spiders they were eating are now gone, the spiders dont keep the grasshoppers and masquitos in check, and then your covered in grasshoppers and mesquitos, you cant eat your pizza when your covered in mesquitos and grasshoppers, so you dont buy pizza, papa johns goes out of business, then the pepperoni farm they get pepperoni from in siberia goes out of business, they lay everyone off, and so the entire population of siberia dies of starvation and heartburn, because all they have is an exccess of pepperonis to eat, so you wind up killing off all of siberia because you released a gravid veiled chameleon in your backyard.
Thats why its not a good idea...