Manga-wise, the trouble that usually goes with getting another pair of eyes is why usually the characters didn't get their spare till they were near blind. Necessity and all.
Cause by taking another Uchiha's eyes were basically nutering them (well, except Madara in the latest chapter... x.
when compared to their old self, unless they get another pair of eyes too.
SL-wise, people don't care as much, so were abusing the heck out of eye trading. To help cut down on that, peoples demanded that the sharingan be more manga-wise leveled that you have to kill another Uchiha.
Them being closely related like a brother is irrevelent in SL, since we all know that the Uchiha incest incident occurred shortly after the incested Hyugans started to die off.
Anyways, the point is,
people abused trading so it became unacceptable.