I have a Viewer.app at /Applications/Canon/Utilities/PhotoStitch/Program/Viewer.app/, which opens .jpgs just fine.
When I right-click on a .jpg and choose Open With from the contextual menu, the list of possible applications includes a bunch of browsers, a bunch of HP and Canon image-editing utilities, GraphicConverter, QuickTime Player, and half a dozen or so others in addition to Preview. It seems unikely that Viewer.app would somehow become the second choice behind Preview without conscious intervention on the user's part, but what do I know?
