Poky, on 2012-February-22, 09:09, said:
With singleton spade, you have a decent defense against 2♦ too.
Maybe. Or maybe I have enough defence to beat 3♦ but not enough to beat 2♦.
Give partner some ordinary hand like AKxxx Kxx xx xxx. How much chance will we have against 2♦ after partner leads a top spade? If we compete to 2♥, we have a good chance of making that, but it's more likely that they'll compete to 3♦ and go one down.