I personally don´t think that it´s a good idea.
Reasons:
1) Deck building with 1 hand possible.
2) I think zanz added a time delay so you can´t add cards really fast (have to find sources)
You see, that you can add cards ingame faster than in trainer. If you want to shorten deck builing time, this would be a way to go.
3) Decks with many cards but little copies (e.g. many rainbows) need more time to be build.