By contextual advertising, I assume you're referring to the Content Network? If so, they advise against it because you don't need it. Context Network ad groups are based on *theme* and not so much individual keywords - Google tries to figure out what the theme of your ad group is by the keywords you've included (and probably the ad group name) and place the ads on the appropriate sites. But it's not necessarily a one to one match with your keywords.
That's why match type and singular and plural keywords aren't necessarily in Content, and also why you probably want to have smaller ad groups with fewer keywords, so as to keep your focus very tight.