One common use for a mesh having multiple UV maps is when combining several
mesh objects into one: the base UV map is the result of joining the meshes
(and will be a right mess of overlapping UV islands), but an additional UV
map is then setup as a copy of the first but with the islands re-packed so
nothing overlaps. The export script now searches for the active UV map and
uses that for both UV coordinates and the skin texture (when none is
specified).