Wow, interesting thread. For what it's worth, here's my take. I've always kind of been in the "no fake tags" camp but you've really made me think about this.
If you absolutely, positively, knew the body number and X code, and were able to make an exact replacement of the original tag, is there any harm to anyone, anywhere, at anytime? Only if they are not made aware of the fact that it is a replacement.
On the other hand, if you don't know the number, it's not really an exact replacement, and it never can be no matter how good the fake is. I understand the aesthetic argument, but I guess the bottom line to me is who are you trying to fool? Yourself or the buyer? Please understand this is not an attack or insult, it's a legitamate question. On my own stuff, I myself want to see the complete picture and can't stand it when something obvious is missing. So I'll put on a part that isn't right and hope I can find the correct one later. And it looks better, even though I know it's wrong.
Anyway, after thinking it over, I think I would put on the fake tag if I was keeping the car but would not put it on if I was selling.
Good, thought provoking question though. Thanks.