I too have a firm belief that no news is good news - if there was a problem, they'd certainly let you know - either the right way through a message, or through feedback. But these cards are also to make buyers aware that feedback exists on Etsy, and is more for the benefit of shops starting out - shops who need the feedback. Bigger shops won't need it as much as their feedback may well be quite massive anyway, so making a sale and having no feedback will go by completely unnoticed - but I still find myself checking my feedback the very day I send out an order!
And as for an increase of feedback, it's working for me! But time is young, as of yet ^^ And besides - Moo has an offer for 100 mini moo cards for free at the moment, so it doesn't cost much to give it a go!