I would assume it's a balance issue. As it stands, Deck Master can still be very useful for consistancy (You want to guarantee a certain start, and don't want to reset 13 times to do it), or with increased starting funds to try a unique tower as the start (Shadow, for example). I know I used Black Widow , Deck Master and Loan Shark one time. It ultimately needed help, but was remarkably fun for about the first 75-100 waves.
He's a hero that gives you +5000 starting gold, but removes your interest bonus and gives -50% Gold from kills. Basically, using him means you have a tactic that requires jumping straight to a Rare, Epic or Legendary card to work. He is also a somewhat interesting pick to use with Money Bin, because the bin will massively overcorrect for his weakness, while still allowing you to keep that starting gold.
How come there are so many towers I don't know about , how do you get them? Are they only attainable from black market or something?
I just got lucky with a Card Forge drop with him (Though I've had less luck trying to get the cards I really want). If you're ever curious about certain unlocks, peruse the "Cards" section up at the top of the page. Andy's really good about keeping it updated, so you can learn about cards you haven't unlocked yet, and think up strategies around them. =)