I think this is missing the point.
Suppose you have two wetland birds, one with a brown power "Draw 2 and discard one at end of turn" and one that has no brown power. You activate your wetland, and there is one bird in the tray you want.
You are going to be drawing 4 cards before your turn is over, and there is only one in the tray you want. Why draw that bird from the tray right away? Draw 3 facedown. Maybe you get 2 really good ones, and now you no longer need that bird in the tray. Draw the 4th bird from the facedown deck, and avoid exposing a new bird in the tray that might be huge for you opponent.
The advice wasn't to draw a single card from the facedown deck, gambling that you will get a better card than the one in the tray you want.
[WHOOPS. I replied immediately before reading the two subsequent posts, and all 3 of us ended up saying the exact same thing. ]