I actually think they create more problems for SEO rather than help. Because I have numerous elements being tested, generally the content is similar, and this creates duplicate content issues. Typically I use ROBOTs tags to hide my landing pages from search engines and prevent dupe content issues.

That being said, whatever it is you learn from your landing pages should be used on your website pages that ARE seen by search engines. Use them to improve your content and offers. This will improve your site performance overall and help your website SEO because of good metrics.