Tony, you truly do know your SEO well, as I have read many of your posts on the subject.
So do not take any offense at what I may say.
If you know how the meta keyword tag was meant to be used, and you used it that way then your not spamming and therefore your not going to be judged by BING.
SEO in my opinion is not about a quick fix or trying to beat the engines at their game. I know that you (Tony) know how search engines view your site. Its kinda like a book....... You have the front cover on a book, we have our heading on the site, (meta area) of the website. Then you have the title page inside, on the website you have the H1TAG.....
SEO is not hard, its the millions of people that make it hard. Do not try to cheat the engines, just build your site with details of what its really about. First and foremost when I build a site its about content to the customer. Then i will use the meta keyword tag to replicate that same content in my meta keyword tag for the engines. Nothing more nothing less.
My SEO standpoint and how I build a website has not changed for 10 years. For what I show up as in #1 spot on Google I have been #1 for a long time and my ranking just not fluctuate. If I fluctuate I would be concerned as that tells me I may have stretched the "rules" and am being judged.