Since you asked, from my experience, there is one very good reason for KO-style functional observables rather than plain JS: immediate naming errors. If I type obj.prop when I should have typed obj.Prop, it simply returns undefined. But if I said obj.prop(), it excepts. This means that naming errors are diagnosed right at the site and immediately. I've found this a significant advantage to KO-style observables, particularly when refactoring code.