That too. Companionship can be one-way. Here are a few things I see:

- bokchoi traps a lot of bugs. Good for nearby brassicas, but not so
good for the bokchoi itself.
- parsnip goes well under tomatoes, despite lack of sunlight. Tomatoes
unaffected.
- collard flourishes under peas (this was an obvious one), but the peas
don't gain a thing.
- squash not affected by beans, despite claims of the contrary.
- lettuce goes well with garlic.

One other thing I have noticed is that brassicas by themselves
inevitably get skeletonized and die (lots of cabbage moths around
here). But brassicas all together survive. And the explnation is
obvious: robins visit a large cabbage patch, but will not check
individual sparse plants. So many cabbages are good at keeping each
other company.