I don't think that's easier, especially from a user point of view. Each product is going to have half a dozen products that we would want to display as related products (whether they are accessories, alternatives, etc.). Creating different classifications for each product, and then tagging those related products with that classification... I can imagine that would be very messy very quickly. Unless I have misunderstood your suggestion, it seems to me we would be creating a classification for each product, and then adding that classification to any products that may be related. If this is the case then a product may end up 50 classifications against it.
I think your suggestion would be ok in a solution where we are just showing the customer some alternatives from the same category, but in our solution we need to very specifically target certain products to show against others.