1) Because the borders between the cases are ambiguous and uncertain. I sent a message a day or two ago (perhaps after you sent this one) which addresses that issue.
2) Because we want to be able to combine works from different sources, As I explained, this desire is usually impossible due to incompatibility of licenses. To reject the GFDL on these grounds and accept some other GPL-incompatible license is a double standard. and something covered by the GFDL cannot be combined with ANY program to produce a work of free software. By now you've seen my response to that point.