I send out email newsletters which will also be published on a wordpress site. The newsletter will encourage the recipients to go to the site to comment, or contribute some way. Would assigning the Author role by default to the recipients encourage more interaction, rather than simply assigning Contributor?
Claiming someone as an author rather than a contributor of something carries the understanding that they did a majority of the work (or at least a significant chunk of it).
In the short term, giving contributors author status may have the desired effect, but in the long term it will most likely cause more problems as you over reward contributors for possibly very little effort. This will encourage very minor contributions (as any contribution gives author status), which is likely to discourage authors from creating content in the first place.
If however you are the author, and your main priority is to have people feel a sense of ownership. But from my experience, any credit gives this whether it is author or contributor.
So unless you have a very good other reason, I would say don't give author credit for minor contributions.