WordPress SEO Plugins should be used by any WordPress powered site looking for high search engine rankings but they are not a magic solution because:
On Page SEO is only one part of the jigsaw
WordPress SEO plugins can help refine a site for SEO purposes but they can’t help if the basics are wrong. Selecting the right keywords to target is of prime importance, they should have a reasonable search volume but not be too competitive and the primary keyword should then be included in the domain name. If a keyword is dominated by large companies with established domains and lots of backlinks it will be extremely difficult for a new site to rank, even if the on page SEO is perfect.
Off page SEO carries greater weight in determining search engine ranking so getting good backlinks to a site is essential and this is something that WordPress SEO plugins can’t help with.
Content is key
Although WordPress SEO plugins can help in several ways they cannot produce content and good unique content, added on a regular basis, is an important part of on page SEO. So if you want to achieve high search engine ranking you should:
Select good keywords
Include a keyword in the domain name
Produce unique content with each page/post built around a keyword.