First of all, I have to laugh at the juxtaposition of "chop and drop" right next to "permaculture for children and old people" in the topic list.
Let's hope the threads stay separate.
Seriously though, I think "chop and drop" has limited utility, especially if you are trying to combat fungal diseases. There are some plants, like pines and oaks, that just love to be standing in all their cast-offs. For many others, it's much better to clean up the detritus under them and give them some nice well-rotted or composted mulch. It all depends on the specifics of the situation.
I think you have a point, but as ever - it depends. Are you, for example, chopping the green manure crop and planting something else entirely where it was? In that case, your concern is addressed, while letting the chop drop where it falls.
Are you looking at chopping something back and letting it grow again? In that case, your concern might be an issue, but it seems to me it is commonly the case that where the plan is to chop and allow to regrow for chopping again, you're already planning on taking that chop to use on other plantings.
I do think you have a point, but that it may be one that is typically dealt with in the ordinary course.
1. Chop and drop perennials where they are. It's their nature to grow in the same place every year, so it may be worthwhile for them to mulch their own space. Seems like that the way nature has it set up.
2. Chop annuals and drop them elsewhere ( near unrelated species) if you're concerned that the current crop might be harmed by the breakdown of the chopped stuff. ie Cutting squash leaves with powdery mildew and leaving them in the pumpkin patch will help spread the mildew to unaffected plants. Leaving squash leaves under the trees might not be such a bad idea though.
3. If you are chopping annuals in the fall at the end of harvest, drop them where they are and plant an unrelated species there next year. This is basic crop rotation and there are lots of different schemes to doing it. I think it's easier to plant unrelated seeds than to move plant debris all over the place.
4. Anything that doesn't show any sign of disease or deficiency is probably fine to leave wherever it is growing.
5. If you have animals, give everything to them. They'll take care of most anything and turn it into High Quality fertilizer.