The idea is that we should be able to share songs with other Spotify users inside of the app. I'd love to open the app and have an inbox full of songs my friends think I would like. The need to leave the app surely reduces these instances. Thanks!
Updated on 2019-09-06
Spotify used to have the functionality you describe, which is no longer present. The discussion regarding this is located here:
Any update on the matter should be posted there.
For more info on how the Ideas Board works, click here. :)