Currently the way export works is it basically needs to boot up Spine, then do the export. Because of that, you have to first run it in GUI mode, input an activation code, and allow it to download the first update. After that you can use it from the command line. This may be an issue, since it means your build server needs a GUI,
I would agree, you shouldn't need an additional license for a build server. Unfortunately you'll need a full version of Spine and activation code though, as that is difficult to change right now.