Before, the value of ITC credits were easier to calculate, understood that they were based on a 10-to-1 USD ratio in order to have practical relevance in the real economic world. Basing a unit of exchange on a real currency is an understandable necessity. That ratio already represented an advantage to US dollar-using participants because it was so simple to calculate compared to other monies.
For anyone who does not use US dollars on a regular basis, the unit of credits was a creative, easy way to translate value into their own real economic lives by focusing solely on the credit value compared to their own currency. Now they are forced to deal with another fluctuating value which makes understanding their own relative currency value more complicated. It also has the potential to complicate users' payments and therefore tax consequences, students and professionals alike. I would not be surprised if this added complication results in a reduction of italki users.
2) The italki loses its sense of equal community.
The change to displaying all values in US dollars surprises me greatly on a professional level as a teacher, translator, and linguist. Italki is a multi-lingual and therefore multi-cultural endeavor. The ITC credits were one extremely appealing aspect of the website because they helped users become part of a single community. A common currency -- as one argument for using USD on the website might state! -- helps create a sense of community and therefore identity. The move to eradicate the created shared space and currency of ITC removes an important aspect of commonality. Interpreting values only in terms of one denomination destroys a sense of shared space, and more importantly, of equal footing. It places the USD at the head of transactions, necessarily implying that other currencies, and therefore economic cultures, are less useful, less valid, less important.