In my opinion, the entire DB package is a good candidate for a package - the ability to use alternative storage solutions is becoming an increasingly relevant topic every day.
I think I would argue that an entire feature, such as Yii's database abstraction layer (of which AR is a subset) is a great candidate for something optional.
I agree, and I would not argue for distributing Yii without the DB/AR package.
What I would argue though, is that if it came in the distribution in the form of a package, that makes it more obvious to everyone that I have a choice - so I can feel confident hitting the delete key and pulling in an entirely different storage layer. A lot of professionals will prefer Doctrine, for example - and in the future, increasing number of developers will probably prefer a graph/document-database.
I think it's important to show that we've thought of this, and to clarify that nobody is forced to accept every aspect of Yii for every aspect of development.
I don't expect to see Yii's storage layer become something you can "replace" from underneath an existing application. But it would be nice if there were no direct ties between a basic web-application and the built-in database abstraction. It's a matter of choice.