Hay there fellow fish keepers. In Australia we have more introduced plants, animals and fish than any other country. I can't think of one that hasn't caused a problem (even your rainbow trout).
Wile we are all responsible fish keepers. There are many people out there that are not. This is why you have snakeheads and carp decimating native fish.
Yes we love our exotics and we keep them responsibly, but how many people out there get something and decide in a years time that its too much work. Flush down the toilet, thrown in the creek or released in the local pond.
What is the potential cost?