After reading these two answers (answer1, answer2) I understood to don't add class instances or critical object instances to class definitions. But I wounder where would I use config? Am I right that I would (only) use it, when I need getter and setter for these properties?
Getters and setters could be one reason, however, Sencha Touch and upcoming Ext 5 use config extensively, in Touch it is even mandatory. So better to get used to it.