In terms of search engine optimization, much has been written about Google and other search engines penalizing sites that feature "duplicate" content. If one duplicates certain content using JMS Multisite extensions, does that eliminate such concerns? Or, is JMS Multisite and its articles sharing extension simply a way to eliminate the labor involved in duplicating content from one site to another? Even if the latter, there is much to commend in terms of the administrative workflow. But it would enable me to better understand just how much duplication of content I'd want to use it for.

JMS and the "Article Sharing for JMS" does not touch anything that would modify the seach engine processing.
SEO ranking depends on plenty factors and in JMS change anything.
Perhaps that SEO will give you better or lower results depending on the context, the domain name, ....

The "Article Sharing for JMS" just give you a facility to display an existing article.
As you can associate it to a specific menu item having a special alias (for keywords), ..., perhaps this will change your ranking.

In our case, we have plenty "jms2win" websites with exactly the same content and the ranking of the other website are low but sometime better on specific keywords.

It will not effect the working of search engine,it will still depend on the quality and relevancy of your content however it reduce the overhead involved in duplicating the content but still remain the duplicate.To avoid this use upto 70% of original content.

If you want to accomplish your business goals, you need to create powerful internet marketing strategies. I am also using this approach for marketing of the blog. Have been using high quality Adwords Management and Facebook ads services. The results are really nice.