If you make decks that have more than 30 cards, then you will not draw all the cards during a match. Therefore, many of the cards will not even be used in the match, and are thus wasted. For example, if you make a 40-card deck, what use are the 6 Devourers in it if you never even draw them?
You can take more than 30 cards only if you have cards that help you draw, such as Hourglass and Sundial. And if you're really worried about decking out, then just take 31 cards, because most people take 30 cards.
If you still disagree with me, go ask some other people. They'll all say the same thing.