As a customer I am with you all the way. It is hard enough as a large or an extra large to see how a product is supposed to fit from a model shot. But since plus size clothing has been re-designed and made from a different pattern it is absurd to think that a size two model is wearing anything remotely resembling the plus size garment (petites has this problem as well).
However, I am in a retail business and understand the additional costs associated with having seperate images for each size range. Since plus size clothing can be a higher risk business to start with not having to take on the costs of additional shots and production can allow retailers to carry more plus sized clothing. It is a difficult trade off for retailers and it is a tough customer experience.