Meta keywords these days aren't likely to help you on any of the main engines, but just for the sake of analysis, let's suppose they have some effect. The essence of page optimization is focus.
But what happens if you add a lot of meta keywords? Effectively, you end up defocusing any focus the keywords tag might have added.
Can it be interpreted as keyword stuffing.
That's what it is, though it doesn't work.
But if the Google, eg, did a manual inspection of your site, chances are they'd notice this as an indication of intent, and they might tend to weigh this against you if there were a lot of other borderline stuff on the page.
The reason I asked is that I was looking at a site which has pages where the text was made of large images interspersed with an occasional html table.
In this case the spider would see very little but the meta keywords and maybe a table. This site has a very well thought out (and large) meta keyword set for each page. So I would think a search engine with nothing else to go on would not be so quick to discount these meta keywords and they are a wonderful way to add a large list of searchable terms.