A big problem I've hit with the Yii-Bootstrap extension is that all AJAX requests are initializing Bootstrap because of preload. This is a huge waste of resources, especially when using AJAX-based file uploaders that split the file into chunks. Large file uploads using that method could be initializing bootstrap hundreds of times.
Sometimes it would be nice if we could run yiic from an action, for example a migrate database button in our admin panel.
Or because you are on a host which doesn't have shell access.
Or maybe you are not allowed run popen or exec.