Canonical domain tag

The canonical domain tag is a new tag that allows different versions of a webpage to be consolidated into one. This is a quick fix for duplicate content problems. I won't go into detail on the tag itself and how and why it works, but I have one problem with this tag... Will the canonical domain tag dumb down SEO/website design?

I have several legacy sites that I look after and they are far from ideal in their design. They have indexed session ID tags, vast URLs, and several other duplicate content problems. Now, I know that I could get the rel=Canonical tag to fix most of these problems and stop pestering the development team on best practices. But I know that by designing things right in the first place will save work in the long run. I have seen Patrick Altoft refer to it as "papering over the cracks" and to be honest I'd rather nag my developers and get the job done properly first time!

I don't want to have to deal with another ancient and unwieldy website, so I will keep to using the tag on sites where development time is strictly limited. I won't make my developers aware of the tag either, in case they decide it's the quick fix to save them the effort of the "best practice" approach.