The majority of search engines today handle singular and plural keywords differently. Were they to treat singular and keywords the same, the search results would be the same, evidently they are not.

Try it now by searching the major search engines for both ‘website designers sydney’ and ‘website designer sydney’, the results are often remarkably different:

Live : website designers sydney »
Live : website designer sydney »
Google : website designers sydney »
Google : website designer sydney »
Yahoo : website designers sydney »
Yahoo : website designer sydney »

It is important therefore, to optimise a website for both singular and plural keywords and in proportion to their respective usages. If most people would search using a plural than more effort should be directed towards optimising the website for the search phrase involving the plural keyword.