Just to add my take on this...

With the times being what they are, reenactors are often the only source that sites have to put a large number of costumed interpreters out at the site.  I've worked at several sites in Michigan that had unfortunately had to cut back the number of their paid costumed interpretive staff, and the weekends that they invited reenactors to the site were the most popular.  The public enjoys seeing a large number of people at a site.  How many times have you been to a fort with only four interpreters?  It's hard for the public to believe that a fort had a contingent of 50-100 men when only two soliders are mustered for a demonstration.  Compensation can come in many forms as well.  Offering publications from the gift shop or powder to replace that being shot off during the weekend makes many reenactors feel appreciated, and can mean more than a monetary compensation. 

The only caveat I would give is that when inviting reenactors to your site, have a standard form that they can fill out that lists safe practices for handling historic firearms, and provide a summary of site history that they can refer to if they are not already familiar with the site (many of us reenactors are familiar with site history, but it doesn't hurt to have a refresher).

That's my take on the discussion.

Jim Sturgill