I personally enjoy repros because it kinda gives me that "what-if" feeling to it. The only one I own is Earthboud zero/Mother 1. and i like the idea of having the cartridge and playing it on original hardware and its almost like it was released. I do agree tho that there should be some kind of small indication that it is a repro.

the only thing I can think of to differentiate between a repro and a pirate would have to be quality, but thats a stretch sometimes. theres really not a big difference.
However i do for some reason find a lot of amusement in the demake category of pirate famicom games, like ALTTP or DKC on famicom.