Yii is the framework, include all more general object.
Zii is a collection of widget, that are not enough general for being in the framework.
For developer this is absolutely transparent. In your copy of framwork simply there is a directory named Zii that contains all Zii widget, and that’s all.
Yii has been splitted for create a raw framework, with only general utilities (AR, url managing, RBAC, I18n and so on), and include all other (GridView, Menu, and other widget) in a separete container.
If you don’t like Zii widget you can avoid to use, sometime they are not fully customizable, but when is possible to use them they are really confortable.
Don’t be afraid of ‘migrating application’. I worked with Yii since the very beginning and the development team has been always very attentive to BC.
Interesting argument, although I am a newbie for Yii now. I think Yii team should be aware of user learning curve. For people who are trying to migrate the application to Yii, at least offers an easy way to archive that goal. Otherwise it takes more time on migrating applications from other framework to Yii, and it will turned down many potential opportunities due to time/human resource limitation. In most of time, painless transfer means everything. If you try to change the way how people thinks and works, at least Yii team should emphasis on and demonstrate how it is better than common practise. That is two step work, while Yii try to promote it as single step to "paradise".
Also, front-end programming and back-end programming will be separated into different group (or handed over to different persons) for complex web-apps, I don’t know if widget approach brings up any puzzle or internal debate in the organization.
I would like to suggest Yii team to survey its users for previous experiences and used frameworks to offer enhanced tutorials to smooth the learning curve.