If plants had immune systems, our lives as hobbyists would be so much easier!
I think it would be quite scary if they didn't. Unfortunately, we only notice it when it does something bad. But it's less that the immune system will fight it off as much as it is that x virus or bacteria simply can't infect the plant, just like how plant viruses won't harm us. I did realize that there might be a way to self pollinate Nepenthes though. They can apparently change sex, so if you took a cutting and that cutting changed sex and was then bred with the original plant, that might kinda count.