1) You seem to be confirming my statement. There is no standard and every kind of file needs its own type of tagging. By the way, the idea that the tags should go inside the file (a la id3) is interesting, but the file paradigm seems to be completely inadequate for it. When you change a tag, the sha of the whole file changes (which is not nice at all). Furthermore, if you have your files remotely, to read the tags you need to transfer the whole file. Which means CO2 and drying up your internet forfait. Furthermore, nepomuk seems not to follow the paradigm of tagging the stuff by adding a nfo file to every dir, but to put everything in its own inexplorable database of ontologies.
2) Maybe just for photos and music if you do not use nepomuk approach to tagging which puts stuff in its own db.
3) Apart from the fact that using nepomuk your 1b does not appear all that natural, my point is this. Say that you use nepomuk and automatic indexing and that you index all your home. Say that (by mistake, during a file transfer, whatever) for 5 minutes a text file full of very secrete passwords enters your home dir. Can you guarantee me that after you erase that file, your secret passwords are not captured for an arbitrarily long time in some index file?
4) When I was administering machines, this was a nightmare. Having users ending up their quotas all the time.
5) Ok, now try to rescale all those images to 800x600. Or say that you messed a few years, so that for every year you want to do year=year+1. The fact is that if you go tags currently you loose scripting and automation. Furthermore, if the tags are in the files (like for photos, music), rewriting tags currently means rewriting an arbitrary large number of huge files to update the tags.
So, I am not saying that tags are necessarily bad. I am saying that they are extremely immature and that trying to /force/ people to use something that is immature (or even giving it as a default) often backfires. Like the gmail labels that no-one I know uses (google is anyway quite good at searching inside messages) or the crazy management of photos on android phones, where everything is shown together even if it comes from 4 different cameras and is copied in 4 different dirs.