If they use an interior page frequently then they have to drill down to it every time they return to your site. Eventually they will stop coming back. Surfers cannot capture the url of interior pages and email it to a friend. This again is a usability issue. The friend may not be able to find the page or may become frustrated enough by the multiple clicks that you lose both customers.
Designers have to make additional static pages to accommodate surfers with older browsers.
This increases development and maintainance costs. Designers have to make additional pages for the search engines to index. Although a couple of search engines have added the ability to parse frames to their algorithms, most still do not have that capability . This costs developers and site owners time, ranking, traffic and ultimately, lost sales. Use frames if you must, but understand the price you pay for doing so .