thorny, you are right about that. Many dangerous agricultural pests, diseases and weeds have been introduced to Australia, where I live, and to other places, by people who had no idea that what they were bringing in was a problem. We need to be a bit careful. If we buy exotic plants we need to buy them from a known clean source, such as a nursery with an import license, which is what I do because I have a collection of rare and unusual vegetables.
On a slightly different note, I have found it useful to share some of my more unusual and rare garden plants with other collectors, both because it is good to have the plants spread around a bit, and to have a source of replacements should something happen to my plants. Yesterday I spoke to an audience of food gardeners at an Urban Farm down near the coast. It was an interesting gig, and a bonus was that some years ago I gave specimens of a number of rare cabbage cultivars to the chap who manages it, and I was able to get seed back of several of them that I had lost in a year in which my garden had suffered heat waves, severe frosts, wallabies and rabbits. This time I will plant them in a better and more protected position. And hopefully soon I will have more seed to share with community gardens and urban farms. Food reserves are a good thing to have!