Can't we just use our email instead of our identity?

I mean now I have two usernames; one identity and one forum username and I can only login with the identity... what did the developers and decision makers thinking?

The system is rather complicated, the objective is to ensure you only have to register once for all of kde.org.
And yes, at some point we will move to using email addresses instead of usernames to login (you'll still need to register once on a site hosted by KDE though).

The current system is showing it's age - at the time it was conceived, integration with the forum among other sites wasn't intended, which is why things are a little complicated.