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.

I do this for a living and it can be different for every B2B client. However, you asked for "most common".....so I'll tell you our most common approach. We typically see a longer sales cycle with B2B sales. We incorporate a combination of marketing automation with retargeting. We will search out industry-specific data sets for our initial retargeting, and then use several layers of retargeting to walk them through the funnel in alignment with marketing automation. The key is Thought-Leadership content. Can you produce this, or are you willing to pay to have it produced? This type of content sets you apart in B2B. Hope this helps!