Would you want merchants to supply you with product feeds with ALL of their products - even the ones that don't have a strong value proposition - or do you want them to give you only the really "competitive" products?
I don't represent all affiliates, but AFAIK the preference is for all products. An affiliate may specialize in discussing the best cabinet hinges and have side information about cabinet tops for example..they wouldn't want just the tops. You never know when a shopper starts looking for one thing and ends up buying something entirely different. If the purchase experience is pleasant, they will be back for related products when they have a need.
More important than which products is making sure that they are properly categorized. Too often everything from shower curtains to staple guns are listed under something like Household Supplies. If your business is all one category, it would be more useful (to you and to affiliates) if it had proper subcategories. Sometimes I spend days to manually fix a feed so that it can be expected to attract traffic.
I really appreciate the comment about the proper categorization. As a merchant, sometimes we look at categorization from our standpoint of "what is the easiest way for me to do this with our shopping cart software?"
Thanks again, and if you have any ore suggestions for us merchants, keep them coming.