Well, the Ideas forum started life as a publicity stunt public relations exercise, and as such was never an important part of the project. It was introduced to counter the general impression that WP development was a closed process driven pretty much entirely by Matt. This was around the same time as the Habari pseudo-fork was launched making a big deal out of being a genuinely open and democratic project.

So yeah, like kvetches and codex and the multiple other neglected corners of wordpress.org, it's one of those historical relics which served a purpose at the time but is now left to moulder. It was always kind of at odds with the 'show us the code' ethos of wp-hackers, and I would imagine that over the years the feedback of wordpress.com users has proved to be a more reliable indicator of what people really want from the software.