At the same time, putting that word out there puts it in the mind of your customers. Now you have an opportunity to educate them! Don't we want people to know why polycultures are better?
The proof of the likely results of running a market garden business and giving potential customers information that only makes them doubt you is in the original post:
"But when I tell people I'm going to do 'polyculture' for a market garden business I get worried looks."
Why suggest to someone looking for business advice that they should do the very thing that is already being a source of friction? You don't want friction in business. You want customers to easily do business with you. Isn't the better way to grow the food, market it, then show/tell them the techniques you are using? For most customers, knowing their food is grown without chemicals is enough. Telling them more when you have nothing to show just makes them look at you sideways...unless you're only marketing to people who are already sold on polyculture or you just like constantly "educating" and trying to overcome misunderstanding, objections, and confusion. I wouldn't think that's a good use of time for a business, but your mileage may vary.