The reason for creating status notifiers, and why Canonical built their appindicator library on top of that technology, was because, quite simply, the XEmbed based system tray icons were broken. They did not lend themselves to a modern presentation, failed to provide a compelling answer for mobile/tablet/netbook form factors and lacked any mechanism to communicate vital metadata to the host visualization. I won't go into all the technical details here (others have covered them elsewhere in the past), but suffice it to say that the old XEmbed system was increasingly becoming a blocker and status notifiers were a big step in a better direction. When we started work on them, it was our goal for the very start to make them attractive to and useful for others so that we didn't accidentally create a "separate silo" technology.
"we wished there was some constructive discussion around it, pushed by the libappindicator developers; but it didn’t happen": I can't speak for the GNOME / libappindicator dev collaboration efforts, but KDE Plasma devs have been working with libappindicator devs for some time now. We don't always agree, but we do tend to get things done and share efforts along the way. In addition, there was a lot of communication about Status Notifiers on the freedesktop.org xdg list where good feedback was offered and the specification improved significantly as a result. So communication really can't be to blame here, at least not communication by those outside of GNOME.
[^] # Re: Hmm
Posté par gnumdk (site web personnel) . En réponse au journal gbirthday : développement d'un "Application Indicator" pour Unity. Évalué à 3.
http://aseigo.blogspot.fr/2011/03/collaborations-demise.html
Ellez, hop, un petit troll sur GNOME au passage: