Taxing opponents is one of the most common practices in the Modern format, and this enchantment would fit into Death and Taxes pretty well — either taxing your opponent for two mana or allowing you to ramp.
In either case, you will be in a good position, since there are tons of draw engines in Modern. As a result, Smothering Tithe will quickly become the bane of the format, as nobody wants to pay so much for every card drawn.
To note, there is another card in Ravnica Allegiance that has a similar effect: Tithe Taker. However, when you have Thalia, Guardian of Thraben in the format, you don't really need another, slightly worse, hatebear.