Good question, David.
My preference would always be to enforce a single URL via redirects wherever possible as this may provide a slightly better experience for users than the canonical tag and it also helps to ensure that all inbound links point to the same URL.
I think the canonical tag is more useful in a situation where multiple URLs are required to effect a slight variation in content (such as ?sort=asc vs. ?sort=desc) but only one version of the page needs to be indexed by search engines.
Of course, the canonical tag could be considered for any of the above scenarios if a redirect is not possible (mod_rewrite not enabled, non-Apache server etc.)
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That's to go with general online retail searches up 12% in the UK YOY # 2013/05/03
Luxury Goods ecommerce rises 25% YOY http://t.co/TSNMIX42Xp #seo # 2013/05/03
Mums are great. We all know that. But they are pretty great at social media too - WHY? http://t.co/6s7f3u2IJb # 2013/05/01