If you have everything showing already and you choose polygons of course its going to hide polygons and show everything else. This is why I say, choose to hide everything first, everything will be unchecked (this is the first part of the script) then you then show polygons (thats the second part), only polys will be displayed, all that is one script. Then you can type in, "showHidden -all;" (of course no quotes) and make a shelf button out of that as well. So now you have 2 buttons, one to show only polys and another to bring back everything else.