Gflo gave you a pretty pointed illustration of the pain this can cause. I'm not a fish biologist, nor did I stay at a Holiday Inn Express last night (I did a couple of years ago, but that's my most recent experience), so I can't really give you a sense of what parasites/bacteria/etc you might bring in. All I know is that I work hard enough at managing my ponds that I don't want to deal with things that I can at least have some level of control over, and parasites, etc. are one of those things I can at least try to control. From my reading here, some diseases can really decimate your fish population, some in varying degrees of painful severity for the fish. To me, it's just not worth it. Is the chance small? I don't really know, but perhaps. Still, it's not worth it to me to jeopardize my paid-for fish.
As to plants, etc, consider what you could be facing. Think of the issues you're facing right now with your various ponds and the vegetation that you've got and that you've asked questions about since joining the forum. Now ask yourself if you really want to take on yet another challenge that is going to utilize more of your resources that you'd like to dedicate to things like fish, feeders, aeration, getting aquatic plants under control. Is it worth your time and resources to add yet another potential problem to any of your ponds?
Just some stuff to think about. None of the bad things may ever come to pass, but then again, they might. That's the reason for the conservative approach that I take with my ponds.