I don't know how they plan to monetize, but plenty of companies do it, such as Mint. When you have all that tax data, I bet you have plenty of opportunity to sell stuff.
Hm, does that mean you'll take a % of the deduction? Hm that sounds illegal actually, but maybe instead the software is free and you charge per hour to get access to a high quality, certified income tax guy to help with weird situations / edge cases / provide a personal touch!