I think it would be better if only one system existed. At the moment there might be a little confusion about where to post improvement, enhancement requests. Both system have advantages. bugs.kde.org has useful categories and I guess developers are notified by email about new posts immediately. While on brainstorm there is not knowing whether they read it at all or not. On the other hand brainstorm allows to edit posts and attached screenshots, mock-ups, etc. On the bugs.kde.org there should be a filter that allows easily access the last added "wish" bug reports.
Being aware of this technical constrains I still think that someday there should be only one unified system. It would be great if more feedback from developers could be received concerning ideas, at least whether developers like the idea or not.
I wish developers would reply and present their point of view on this issue.