One of spotify's best selling points is how it lets you discover new songs. It's what drew me personally to Spotify and it's the sole reason I stay.
The suggest song feature is one way it does that. At the bottom of every song with less than 15 songs the AI shows a refreshable list of five songs that are relevant to the genre of the playlist. This is a terrific feature and it's severely underutilized.
Firstly, I see no reason why it should only be limited to only playlists with less than 15 songs. When you get past that threshold the feature just vanishes and that's incredibly frustrating as so far, it's been my main source of new and relevant songs.
Playlist radio is not an equivalent feature to it as it contains songs that are already in the playlist. In fact, it's worse with larger playlists since that means there's more songs that are already in the playlist which means more skipping to find newer songs.
The aim should be to find completely new songs and the suggest feature does a fantastic job at it. It could be used for so much more.