Reasons not to have a clustered index in SQL Server 2005

Question

I\'ve inherited some database creation scripts for a SQL SERVER 2005 database.

One thing I\'ve noticed is that all primary keys are created as NON CLUSTERED

Answer 1

Here is another (have it already been provided in other answers?) possible reason (still to be understood):

• SQL Server - Poor performance of PK delete

I hope, I shall update later but for now it is rather the desire to link these topics

Update:
What do I miss in understanding the clustered index?