Some search engines don't consider the META tags important or give it very little consideration (Google).
There are still alot of SE that you can optimize your meta tags for (yahoo, msn, altavista etc).
The reason it states you to lower your keywords is because many search engines will not read past the a certain amount of characters/keywords and making the irrevelant. So makes sure you add your most important keywords up front. - Don't repeat a certain keyword / keyword phrase more than 3 times.
If you add more, No harm is done. The goals is to target the main focus points of your website = 20 keywords.