Title says it all... but it's worth repeating: Model Rules are not often used by Rule authors.
Instead, Rule authors generally work with Designs as they build applications to be run by end-users, in a custom user interface. When end-users make changes in a custom UI, that interface spawns models that instantiates new entities based on the Designs created by the rule author.
During the times that rule authors work with Model Rules, they usually are working with Models to test and debug how things will behave for end-users. In that respect, Models offer you an excellent means for testing.
