The catch is that they're only available for the duration of your session. They are, however, available in all apps across the system.
Someone asked about how to do it on Facebook this week, and at first, I just pointed them at the install script for PowerLineFonts which loops through all the fonts in a folder and install them.
I've used this more than a few times to install some fonts, including the PowerLine ones, which are great:
$sa = New-Object -ComObject Shell.Application