The short answer is content and search engine optimization. If your IDX website provider installs their IDX solution on your website through the use of frames, search engines see all of the content, but give you no credit for it. The way they see it is, your website has no content and your provider has great indexable results. This gives them (the IDX provider) a huge advantage for SEO. If they sell 100 websites with framed content, that's 101 websites that give them search results. As you can see, it's in their best interest to sell you a framed solution. Be wary, be smart, and own your own content!