I don't claim that there's anything original in what follows, but I'm in a vicious mood today and this really set me off.
As I was driving today, I got cut off by an SUV with a "baby on board" sign hanging in the rear window. Ordinarily I don't even notice the damned things anymore, though they never failed to annoy me when they first appeared. What exactly is supposed to be the point of these things? Am I supposed to look at your sign and think "OMG I should be extra-careful not to take any risks that might result in my death or maiming, or the destruction of the single most valuable piece of movable property that I own, because your special little brat is on the road?"
Even having been a parent of babies myself, I still find it impossible to imagine a motivation for displaying one of these things that doesn't make the displayer a total wanker.