I personally have no problem with it.  I'm not saying I support pirating games in all situations, but does anyone buy Nes games anymore? Other than collectors, of course, but the original cartridges aside, does anyone buy Nes games to actually play them? All i'm saying is, if the game isn't making any revenue anyway, should game developers release the game for free?  
 
Sorry about asking so many questions. I say yes, if they are: 
 
-At least 10 years old 
-Have sequels or successors 
-Are no longer in production 
 
The point is, by releasing a game (that wasn't selling) for free, it could build up an audience it never would have gotten otherwise.  
 
Don't get me wrong, i'm sure this won't actually happen,  but am I missing some big reason game developers don't do this?