In the past few years, I don't think a week has gone by without someone exhorting me to sign an electronic petition of some sort. Usually these are excellent causes and very well-meaning people who want to change the world… so long as it doesn't involve actually having to put pen to paper, go anywhere, etc.
I've seen little evidence that people take these electronic petitions very seriously. I suppose there's little to lose in signing one, except that it may substitute for "doing something" in the minds of people who care about a cause, and it replaces actually writing a letter, making a phone call, or even protesting or showing up in person to make a statement. Granted, you'll only be able to mobilize a small fraction of the people who'd sign an electronic petition, but that small fraction may make more of a difference than a mob of digital signatures.