While I might agree with you in principle, I think it would be unwise to make such an assertion. If "animal rights" is an unworthy cause compared to other, greater ones, then we're saying that the greater the scope of the problem, the more important it is, which sort of negates the "ground-up" approach to solving certain problems. More than that, we could similarly say that curbing domestic abuse is a waste of time when there are children starving in North Korea. I think domestic abuse is a pretty nasty thing, and while it might not be as glamorous as famine relief or stopping terrorism, it's probably a worthy thing to fight.
No, we shouldn't be saying that "animal rights" are insignificant next to greater causes, we should say that "animal rights" are an insignificant cause, period. But then again, I've always held views of the Benthamite strain in contempt.
Besides, it's funny how similar Hamas and PETA are. Both have a social / political wing, and both have a militant / terrorist wing (in PETA's case, the ALF). It's a sort-of amusing version of the "one man's terrorist, another man's freedom fighter" problem. PETA doesn't view Hamas' goal of jihad or palestinian revolt or whatever in high regard, and Hamas' doesn't give two ****s about the poor widdle mouses, but they hold in common a shared view of necessary and just violence against civilians.
Theres' no reason they shouldn't be able to say what they want, there's plenty of reason that they should be ignored.