Actually, this is a bit of a binary question. Meta data is certainly not bad for you and just because Google doesn't specifically deal with Meta Keywords, doesn't mean it ignores them! There's also no limit and there's not a lot of evidence to suggest a penalty, except where it's clearly (to a computer system/algorithm) being abused, like irrelevant words and stuffing. Google specifically deals with 10 or 12 types of data in a file that we know of for certain - Description, content, links, Page title, index, canon etc. If and when it wants to. We know that just because you specify a canon or page description, it doesn't mean that Google will always use it. It doesn't.
But to say that because it doesn't filter a Meta Keyword specifically is not the same as saying it's ignored. So don't mind the spammers and the fluffers - use keywords and be happy.
As for Bing (and thus Yahoo), they have in the recent past specifically mentioned 2 keywords per page on their Webmasters blog.