I think SitePoint's lack of previous page links is down to it's target audience. Web designers generally know their way round a web browser and it's not really necessary.
Don't forget though, the previous page of an article might not be the last page the user saw. For example, articles that are split into several HTML files might get indexed by search engines and a user could jump into the middle of the article. I've found it's sometimes worth having a series of links at the bottom allowing the user to access the various bits of the article from any page. Like those little arrow things at webreference.com.
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