1) Hate Crime Hoaxes galvanize special interest groups - but behind a lie. There is an inevitable sequence after disclosure of the hoax. First the attempt to cover up the group's participation and complicity, then the attempt to distance from it (for example by claiming all efforts were in support of "all victims" not just the hoaxer,) then highly focused anger at the perpetrator to excuse any personal participation and finally a silencing effect while the group attempts to have it all go away. The end result is profoundly divisive.
2) Hate Crime Hoaxes expend a huge financial cost in law enforcement, investigation and emergency response. We all pay.
3) Hate Crime Hoaxes extract money and good will from kind-hearted people who will be less likely to trust and help others who are genuinely in need in the future. Being conned is an experience that never quite leaves you.
4) Hate Crime Hoaxes create a dishonest relationship with the media as journalists and editors who took the bait do not wish to be associated with the fallout of the big lie exposed. There is always vastly less exposure to the Hoax than there was to the Hate Crime itself. Because of this some people never learn the hate crime was a lie and continue to pattern their assumptions in life under a false banner.
5) Hate groups love Hate Crime Hoaxes. What better proof does a atrocity denier need to claim that all victim reports are false than to point to a false report exposed? Haters feel free to point to the exposed lie saying "See, people of that (color/religion/gender/nationality/orientation) always lie!
6) Hate Crime Hoaxes have an agenda, and that agenda is to manipulate public sentiment. Whether the end result creates further hatred towards the accused perpetrators, or when exposed, towards the special interest group pretending to be victimized - there is ultimately just more hate.
7) Sometimes innocent people are charged and even convicted of the hoaxed crime. In each case we have seen recently the hoaxers have actually pointed their fingers at neighbors, dorm-mates etc in attempting to substantiate their lie. There's a reason why "Thou shall not bear false witness" is a common and important religious edict - the damage to the fabric of society by punishing the innocent is extreme.