If you have a jury of 12 and require a unanimous verdict for conviction, then 46% of randomly chosen juries will not convict people for the aforementioned crimes no matter what the evidence, because they will include at least one person who believes those acts should not be criminal.
That spits in the face of the notion of equal justice for all. You and I commit a crime together, but have separate trials. The evidence is the same for both of us, but one of us is convicted and one not, because one of us happened to get one of those 5% who thinks stomping blacks or gays who get out line is OK.
Ugh.
Another big problem is that once you tell jurors they can ignore the law in order to acquit, they will figure out they can also ignore the law in order to convict. Bogus convictions won't be as frequent as bogus acquittals, but there will be some. (And you can't count on the judge throwing out the conviction in those cases, because the judge won't be able to distinguish those convictions from those where the jury simply believed the prosecution's evidence and witnesses over the defendant's evidence and witnesses).
Our system is designed around checks and balances. How do you provide checks and balances for the nullification power? The only one that anyone has been able to come up with is to not tell the jury about it. That way, it only gets used in cases where some juror recognizes that not only would there be an injustice in applying the law to the case at hand, but that there has been a breakdown of the system making it so that this injustice will not be addressed elsewhere, so that the juror decides that even though he has sworn to uphold the law, he must break that oath.
Nullification is almost never appropriate for a marijuana case. Even though I think marijuana should be legal, and I know all about nullification, I would never use it there at this time, simply because the system has NOT broken down in this area. Want to stop people from getting convicted of marijuana crimes? Then elect legislators who will decriminalize marijuana.