Sometimes, it is necessary that only one instance of a non-persistent entity should be associated to a persistent entity, for example when you want to dynamically load configuration settings in-memory based on a(n anonymous) user's session and have those config settings only live as long as the session lives but still want to be able to retrieve those settings through $currentSession.
However, the requirement of associations between persistent and non-persistent entities having a multiplicity of [1 - *] means that every time you need an associated non-persistent entity, even if there is always only one of them, you always first have to retrieve a list of the non-persistent entity and then filtering that list on the first and only instance of the entity.
Also, you would always have to make sure that, when instantiating the non-persistent entity and tying it to the persistent one, there are no other instances already associated (it could happen by mistake), and you will have to make sure all existing instances are removed before setting the association.
All of this could be avoided if it were possible to use [1 - 1] associations between persistent and non-persistent entities.
True