Often presentations are required to meet the brand-standards of an organization. That includes colors (which are robustly supported in Prezi) as well as fonts. How can we meet brand standards if there is no support for custom fonts?
However, the “answer” to the question states “If you have a specific font that you would like to use, it is possible to use fonts that are not within the Prezi library.”
There is a link to the knowledge base in the “answer,” yet the knowledge base states "For more advanced options with fonts, click the ‘Advanced’ button on the bottom of the theme wizard. From here you can fine tune the color of your font using RGB code as well as select from the library of fonts that Prezi provides.
The operative phrase there is “select from the library of fonts that Prezi provides.”
Frankly, 16 fonts does not a “library” make. There needs to be support for custom fonts or at least a larger selection (read: 100+) of usable fonts available.