Usually Google and Bing do a good job of figuring out when terms are similar unless they have completely separate meanings (see Rand's response on singular vs plural). In your case if you go to google and type in "switchplate" it actually shows the "Did you mean switch plate?".
Also if you do a search on "Switch Plate" you notice Amazon comes up first with the word separated.
So those two notes are some big clues as to what direction to go for that example.
Either way I would use PPC as a playground here and run exact match ad campaigns for both versions of the term, find out which gets more traffic for you, which gets better conversions, and then optimize for that one first. Go with the data.
Anecdotaly I have heard singular is better for e-commerce as plural is usually used in language when talking about an item informationally. So I would probably go for singular and throw in a couple of plural versions on the page like an h2 or h3 link for "Other switch plates".
This could also be handled by doing your high level category pages as plural, and specific product pages as singular. I would probably keep a term like "switch plate" consistently separated in my copy though if I saw this was the big winner in my PPC test, but that is the copywriter/editor in me begging for consistency and not the SEO tweaker going for every version. In the end I still want my site to be user friendly and clean.