Everything should be iterative.

In software, everything should be iterative. Not only that, but the size of the set being iterated over should never be statically set in code. Functions and methods should never return a multiplicity of one. The exception in this case would be if the function simply performs some action and we are not interested in the result.

This is the backbone of extensibility in software development. As soon as you impose multiplicity limits on objects, you are essentially eliminating any chance for extension in that component. If you have some method of some object that returns another software object that is not iterated-over, the chance for that method to return a dynamically determined number of is gone.

What if the behavior caused by the result of some method invocation should differ depending on the length of the number of objects returned? This is seldom the case and you can always have alternative flows of control depending on the length of the result. This extra bit of code in extreme cases is a small price to pay for the extensibility this patter offers.