I've never been to one. I can imagine the same mixed feelings, esp. the expo's like Tinley which seem to vendors for "everything under the sun." I would LOVE to see all those animals in person, but I'd also worry about bringing home mites, and diseases my birds could catch. I also worry about the quality of the homes those animals are going to end up in. I have a dim view of most of humanity. Even most well-meaning people are generally clueless.
Bird fairs are VERY frowned on by parrot keepers, again both for bio-security, and the stress on the animals. I can't say I usually know what my snakes are thinking, but the birds are better communicators - I hate to see animals visibly upset.