Currently Alexa's Voice Services API requires some sort of audio output for its replies, but there are some devices that do not have audio output capability. If the API allowed written or visual notification output, then these devices could be used with Alexa. The Pebble watches are a perfect example. They have microphones for input, but no audio output (no built-in speaker). If they could show written reply output on their display, then they could readily be used with Alexa. As the API is currently, they cannot use Alexa at all. The only workaround available is if they output audio replies via the phone they are paired with, but Pebble doesn't look like they are interested in that route.