I know as sound cards get bigger and bigger, manufacturers make the switch to power adapter rather than bus powered.
What's the justification behind that?
Why, and for whom, is it better to have AC-powered audio interfaces?
USB power is limited to 5v @ 500ma - this is about 2.5W which is not a lot, particularly if you are feeding a few channels with phantom power.
For smaller output only interfaces, USB power would probably be fine, but once you are powering input preamps and phantom power, perhaps with a headphone monitor, you need more power than a USB interface can supply. Also bear in mind, that if you are using a passive hub, that power has to be split across other devices - dongles, hard-drives etc.
AC/DC power adapters provide the opportunity to supply the device with significantly more power than is available from the USB interface.
The last thing you want is power-related recording dropouts.