The behavior in multi-select custom fields it is done through a selection box. It would preferable if multi-selection were accomplished in the same way as it is for the "Affects Version/s" and "Fix Version/s" fields. This widget gives one easy access to the currently set options and affords easy removal. In the selection box, it is more difficult to scroll through the options to find what has been selected and more prone to error when deselecting or selecting multiple options.