I've heard from quite a few people that adding keywords in a site's meta-tag is not important for SEO, why do they still have them then and should I worry about them?
It's true, in general Meta Keywords aren't used by search engines to help rank pages, in fact Google's own SEO genius Matt Cutts explained that us back in 2009. So why do some SEO consultants still use them?
Some might say that perhaps, in the future, Search Engine algorithms may start using Meta Keywords again, so it's better to be safe than sorry. This is true, though highly unlikely.
For us, we like to use them to help write the content on a page. In general, pages should only be optimized for a small number of keywords. When we sit down to help analyze a site and look at the content on the page, or write the content, we like to come up with a short list of keywords for each page that should be optimized. Putting those in the meta-keyword spot helps keep those in mind when we're coming up with the titles and copy for the page.
They aren't used by search engines, that's correct, however they are useful when looking for terms to advertise on for the page, and keeping in mind what to write.
Keep in mind that the meta-description is also not widely used to help rank pages, though it is used on Search Engine Results pages, and Bing does care about the description. So in general best practices, it is good to keep the title, keywords, and description filled in, even though in the end, the only meta-tag that is used in determining a page's ranking with search engines is the "title tag".
For more information on which Meta tags that Google cares about when crawling your site, take a look at their list of "Meta Tags that Google Understands."