They aren't. They're avoided in SEO since even the smartest Search Engines can't read what's inside a frame. Don't use them.
I'll agree that frames should be avoided in website development. Period. However there are ways to design framed pages that will allow for spiders to be able to index your site. It certainly adds overhead to the design process and I would never recommend anybody do it, but sites that use frames can achieve indexing and decent SEO when substantial care is taken. Here are some articles that you might find worthy of reading about the topic: Do Frames Affect Ranking? SEO for Websites using Frames How to Optimise Framed Sites