1) It creates a climate in which those who have legitimate objections to Israeli policy are intimidated into silence, lest they be accused of bigotry otherwise.
2) It makes it so that ACTUAL anti-Semites who choose to use criticism of Israel as their cover are better able to get away with venting their venom - since, after all, many who aren't anti-Semites will secretly wonder if this person is really a bigot or just another victim of an over-zealous pro-Zionist crowd.
In my mind, there is a very simple way to tell the difference between a legitimate critic of Israel and an anti-Semite - i.e., whether the position articulated by the individual in question focuses on the actions of individuals and institutions within or pertaining to the State of Israel OR whether that position attempts to, by extension, make larger comments about "the Jews" in general. The litmus test may seem basic, but in my experience it works like a charm.
That is why I resent it when many of Israel's more outspoken critics on the left (Jimmy Carter, Dennis Kucinich, Michael Moore) and on the right (Ron Paul, Rand Paul, the late Barry Goldwater) are denounced as Jew-haters. It is also why I feel, just as strongly, that comments like the one made by Helen Thomas a few weeks ago ARE undeniably anti-Semitic.
For further elaboration on this position, I cite an excellent editorial in The Washington Post written by the rabbi whose interview prompted her shocking statement:
She didn't say that the blockade was unjust, or that aid was not getting to Gaza, or that there was a massacre on the high seas, or that East Jerusalem is occupied, or that the settlements are immoral . . . and get out and go back to West Jerusalem, Tel Aviv, Haifa and Eilat. No. This was not the two-state solution. This was get the hell out and go back to the places of the final solution, Poland and Germany. The Jew has no connection with the land of Israel.
This one isn't even ambiguous.
At the same time, you can't be too harsh on those who are reluctant to fully accept that Helen Thomas is a bigot. In part this is due to her sterling reputation as a White House reporter who broke gender barriers, asked tough questions, paid severe prices for her bravery, and has been around since the days when Richard Nixon was a mere vice president. Yet in part it's also because, at the end of the day, groups like AIPAC - which are too quick to play the "anti-Semite" card whenever it suits their geopolitical agenda - have created a latter-day "boy who cried wolf scenario". We need to more vocally condemn the practice of branding Israel's critics as anti-Semites, not merely because it stifles one of our nation's most cherished precepts (the right to free speech), but because it makes it that much harder for us to be believed when a real anti-Semite rears her ugly head.