Also, this thread is clearly full of very sensible, reliable and safe drivers.However in the real world, not everyone is.
If a yellow, plastic sign makes parents feel more comfortable, great.
If it reminds idiot drivers that there are in fact humans, especially vulnerable humans in the other cars around them, then great.
If it does absolutely nothing, then no harm done.
You're right no harm done either way, but I also think parents who think it makes a difference need to give themselves a bit of a reality check. When you think about it harshly it's illogical.
Idiot drivers drive like idiots regardless of the car in front. They're certainly not going to think "I was going to rev my stupid engine on my penis-extension-mobile so the world can worship my custom exhaust, but now I've seen your princess is on board in front I won't".
The rest of the world, from the good drivers to the nervous drivers to the not so good but we'll meaning drivers, aren't going to think to themselves "I must do something special because there's a baby in the car in front. Ordinarily I drive safely from A-B, but now I need to be extra special super duper careful because Tina and David, who I don't know, have popped a sprog out. If they were taking David's mother to her hospital appointment then that's different, I shall channel my inner arseholery because no fucks shall be given for Brenda. If they want careful and responsible driving, they better put a Brenda on Board sign in the window".