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The thing to keep in mind is that for every other phrase you put into the keyword tag, there are probably many more pages within a SE database that contain that phrase in a more prominent place, like the title, or the first part of the body. Even if the particular engine factors meta tags, it will only return your page after all the other pages that contain the phrase in the title have been returned. That being the case, there just aren't to many situations where you'll get a great deal of benefit from a lengthy keyword tag.
Use the KWT to first reinforce the phrases contained in your title, and then add additional phrases that actually appear on the page. If you have many phrases that are relevant, you need to start brainstorming on how you can develop additional content to target them.