Here are a couple of ideas about meta keywords from Google, Bing, and Yahoo that I found online. Here's what they are about to say:
Google's Standpoint: Not Used
Google has mentioned they no longer utilize meta keyword tags in their algorithm for search rankings. Google did say that they will not guarantee any perminence in this decision in that there is no promise that it will never be used again, however, they did say it was highly unlikely they will use meta tags as a deciding factor in the future. There are still individuals who believe meta keyword tags can have negative ranking effects by using them on Google, however, I have been unable to substantiate this in my findings or find anything official that backs up this theory.
Yahoo's Standpoint: Rarely Used
What changed with Yahoo's ranking algorithms is that while we still index the meta keyword tag, the ranking importance given to meta keyword tags receives the lowest ranking signal in our system.
Words that appear in any other part of documents, including the body, title, description, anchor text etc., will take priority in ranking the document - the re-occurrence of these words in the meta keyword tag will not help in boosting the signal for these words. Therefore, keyword stuffing in the keyword tag will not help a page's recall or ranking, it will actually have less effect than introducing those same words in the body of the document, or any other section.
However, when no other ranking signal is present, unique words that only appear in the meta keyword tag section of documents can still be used to recall these documents.
In Summary, Yahoo accredits virtually no weight to meta keyword tags. If there are no other ranking factors on your website other than meta keyword tags then Yahoo will use the tags as a last resort for your pages ranking.
Bing's Standpoint: Not Much Weight
Bing has affirmed that they do not support the meta keyword tag and have stood by it for a very long time until Duane Forrester, a Project Manager for Bing posted the following:
I'll make this statement: meta keywords is a signal. One of roughly a thousand we analyze.
Getting it right is a nice perk for us, but won't rock your world. Abusing meta keywords can hurt you.
After seeing this conversation, it is apparent that while Bing is subscribing to a similar view as Yahoo in the fact that it does not put much weight on the use of meta keyword tags, you do run the risk of them being used against you as a negative ranking factor.