"Why," I get asked all the time, "won't you sign an NDA? Are you going to steal our idea?
This dialog is in full swing in a conversation started by Brad Feld. Here is Feld on "Why VC's Don't Sign NDAs".
I'd add that in adddition to creating entanglements, they suck up an inordinate amount of time and effort for no benefit. I can't think of a single good reason to consider one.