I've been doing a lot of research on this and haven't found a good reason why not to plant veggies together.
Unless you are saving seed for next year, of course. That's not something I do. If you are saving seed, the recommended distance is 20 feet. An example would be keeping tomato species true to form like Heirlooms.
Most of the reasons why not to plant close together seem to be wives tales. Now some plants are more suceptible to certain diseases or bugs and that's why the wives tales started. If the two or so suceptible plants got sick or infested the other would also. There's so many more helps for those kinds of problems now, I'm not worried about it.
The people on the Vegetable forum helped me out and there is a post there if you'd like to look.
It all depends on if you will be saving seeds is how I took it.