Why use Cross Sells?
You can 
offer products to supplement or complement your customers’ tastes while they remain on the page where they were shopping.
Yahoo! Store does have a built-in cross sell feature, but it resides in the shopping cart. This means the customer has already made a buying decision and may be thinking about money before they ever see a cross sell option. Your Store Addons places its Cross Sells on the item body so the customer can see how well the products go together before price becomes a consideration. You now have the opportunity to sell better products that may actually cost a bit more.
Your Store Addons provides the ability to specify cross sells for an entire section at once. This is helpful in two ways. First, it lets you suggest general cross sell items to a customer. Second, it can buy you time until you decide to specify your cross sells on an item by item basis.
Cross Sells from Your Store Addons allow you to more fully interact with your customers in the easiest way for both you and them.