I would really like to see such an important control become part of the core, mostly because I dislike using user contributed extensions in a framework I am not fully capable in myself. More importantly though, the API documentation is lacking (probably because it's a third party extension?).
I used the control with some success when I first started with extJS a few months ago, but had issues with reloading selections or data in FireFox or something and I gave up due to my inexperience with ExtJS, opting for a grid with multiple selection checkboxes. While this works it requires me to manually restore the checkbox state and just feels clunky when compared to the multiselect.
Just a suggestion, seeing as I cannot seem to post suggestions in the suggestion forum without becoming a commercial user, just yet.