Q:

Why unidirectional one-to-many association on a foreign key is an unusual case and is not recommended?

At link http://docs.jboss.org/hibernate/core/3.6/reference/en-US/html/associations.html#assoc-unidirectional-m21 it is said that 

A unidirectional one-to-many association on a foreign key is an unusual case, and is not recommended. Instead the documentation
  recommends doing a  unidirectional 1-to-M using a join table.

I am not sure why documentation says it a unusual case and not recommended. Because as per my experience it is very common case like customer
can have many account in bank so there will be one-to-many relationship between customer and account where customer will be having association
accounts which will be collection of account for that customer.
So why we need to introduce the join table in this case?

A:

What @Alex said was correct. There are two parts to this answer.
I too feel that the case is not unusual. However as per your example if a Customer has a Collection of Account supports the scenario. However in the example Account CANNOT exist without a Customer.. Consider another scenario where the collection item  is independent of the Owner.
Let say the relation between Owner and Car. Owner has many Cars, hence a one-to-many relation between Owner Entity and Car Entity. The database structure would be
Owner : [OwnerId, Name...]
Car   : [CarId, Name...., OwnerId]

Here we assume that Car CAN exist without an owner. In such a condition there will be Car records with null OwnerId. Existance of null degrades the information. By degrade I mean the quality of information stored is reduced as there are unknowns. The above structure can be re-modeled to
 Owner    : [OwnerId, Name...]
 Car      : [CarId, Name....]
 CarOwner : [OwnerId,CarId]

Thereby we avoid having null valued entries in Car and the association between Owner and Car is well defined.
(as @Alex correctly pointed out)
In Hibernate one-to-many relation has no recognition of a join table Hence we use many-to-many with unique flag set to true to map the relation. However mind you introducing a join table will unnecessarily increase the number of joins.
Hope this explaination helps.

