Fonts are not included in the apk. If a device for eg. Nexus 5 has roboto set as the default font it will be used. I think this is a manufacturer issue.
Stock Android will be default in the future (betting on that). Anyway, would like to use Roboto but setting it through code feels like a lot of work. Because most Androids will support Roboto in the future I think it would be "nice to have".