...And the latter ("the possibility to listen to a song entirely") is very useful, see for example JoAnn's comment. Also for our thousands of daily users who listen and compare versions.
Unfortunately Erik is trying to frame the discussion as "Spotify is not reliable, hence...". This is not basic idea of this project, nor was it presented it that way.
General answer, Bastien: Don't get me wrong, I have no animosities against Spotify. It's a really great site for finding and streaming music (and more). The problem is their being pushed at every SHS level. Some reasons, also to answer to others:
At least for non-subscribers, clicking on Spotify track links added as external links to our 'performance' pages redirects to Spotify release pages, often enough not going for the 1st releases we attributed the recordings to.
Next step, on such release pages clicking on track or any other links leads nowhere but to a sign up resp. in banner. That's it. No possibility for non-subscribers to on site listen to anything, let alone full-length tracks.
Spotify links on 'performance' pages at least result in 20 sec. 'video substituting' listening snippets. Such will (only) play full length to Spotify subscribers simultanously being logged in over there, though. At least that's what Mathieu explained.
Besides non-subscribers apparently hardly being able to identify versions on Spotify at all: If not verified by 'high(er) class' sources that tracks were indeed 1st released as they attribute them, we, even by listening, can't be sure if they are previous B sides, re-recordings or whatever.
The above may go for (a. o.) Apple Music and YT, too. But (if available), YT clips will always run full lenght for everybody. That's actually a main difference.
And, as a big share of editors and CCs for good reasons refrain from adding Spotify links, I'm still wondering why there is a Trac ticket for in the editor stats mirroring the number of Spotify links added. Or did I misread that one?