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In terms of Search Engine Optimisation, I haven't seen any categorical proof that active or passive tone will make a big difference. When it comes to writing style, the use of an active voice is encouraged in journalism, copywriting etc. It's much more effective at encouraging users to take action (i.e. buying a product for example). Whether or not that should be included in the Yoast plugin depends on personal preference. Personally I use the comprehension scores and anything grammatical as a rough guide, but nothing more than that. Especially when WP still insists on American spelling checks, and most of the subjects I write about tend to use fairly niche technical terms which will be flagged up every time they appear as a spelling mistake, and as potentially too complex for everyone. I'd say it's always worth testing on a couple of products if you're unsure. Personally I would try and phrase as much as possible in an active voice anyway (Especially any Call To Action), but if, for example, I wanted to echo Wikipedia or a scholarly academic article, I might choose to go more passive... Any automated checker will only ever be a reasonable guide - and I say that as a big fan of the Yoast plugin - there should always be human judgement applied to whatever is recommended...