"I thinik it's reasonable to criticize Derrida, but are ideas themselves dangerous?"
I'd say some systems of ideas contain inherent self-contradictions which, if the ideas are applied, are intrinsically destructive to the socio-cultural fabric (one might think on analogy with a computer virus); and I'd put deconstruction in that category.
Fortunately, even deconstructionism is just another text.
Ideas don't kill people, people kill people. As ideas are always contained within a brain, no idea has ever been destructive to the "socio-cultural fabric." It's how people act upon ideas that causes the problems.
I think post-structural analysis is like a tool. It actually can be applied from the right (in our political context). For example, we can deconstruct the Democrats by finding the nexus of power on a given policy, and then find their polarity. We can find a reference to our own position in that context and shred theirs without even saying what we're doing.
Furthermore, I would argue that it is the application of post-structural thought that leads to self-contradiction. I found arguments around the Internet suggesting that Derrida accepted external reality; but he went much further than supplying us with tools to rethink the world (the interior of which was more interesting to him).
I don't think he'd complain if we said he wanted to destroy the west with his ideas. And I doubt he or Foucault would be surprised to find out that we can use them to intellectually destroy his apostles today.