I have thought about it
but never went through with it.
I would be most worried about introducing alien things into my tank. Not only pathogens but many things are harberd in plants, like eggs of snails and who know what else. I would rather spend the money and know that the plants I have in my tank are safe. But on the other side, I have gotten snails from pet store plants and I'm sure they are not completly safe either. So if you know of a river or stream that is exceptionally clean, and it has plants in it you could try. There will always be a risk factor.
About the water temp. I would think that this would be the biggest problem, but I keep outdoor pond lillies in my aquarium that I got from my moms pond (we live in MO, so the pond freezes in the winter and the lilys still live) and they actually grow like weeds in the warm tank water!