I've just run the Keyword density analyser v1.1 and noticed that no differentiation is made between single and pleural spelt keywords and phrases. In fact all the single entries seem to be ignored, with only the pleural versions listed.
Do all the spiders work this way? or is there still any reason to include single and pleural spellings of your key phrases and words ?
You definitely want to target singular and plural versions of your key phrases separately.
For a good example of the difference an "s" can make, you can spend some time at Google doing some comparative searches. If, for example you search for the word "apple" you get a completely different set of results than if you search for "apples."
The single word "apple" will produce a bunch of listings about the computer company, while the word "apples" produces a list of sites about the fruit. If you drill down to the 5th or 6th page of results in the "apple" search you will finally see a site about the fruit, but you will also see that Google doesn't highlight any ocurrence (in either the title/description or the cached page)of the word "apples." The site about the fruit is listed based only upon the ocurrence of the singular version.
Obviously this is somewhat of an extreme example because there are so many sites on the web that are about Apple computers. However, even if you do searches with two word phrases that are common to only one niche (something like apple recipe and apple recipes) you will see a similar pattern.
The trick is to figure out which version your target audience is most likely to search for, and then build your content around that version.
Spiky, any chance I could see the page you tested for reference? My analyzer over at SEW, should be making differentiations between singular and plural words. I've not put any fancy stemming into it at all. So, either we have a communication gap here and I don't understand, or there is something a miss with the analyzer. Try running it on the webmasterworld home page once and see if it works the way you think it should.