In short, the simple functions are the lightweight versions that have some advantages in certain situations:
I.e. the simpleQuery allows you to execute SQL statements directly and can retrieve result sets as well. However, they are not very useful in binding with data-aware components. They do not support bi-directional cursors so you can use them in a unidirectional dataset only. In PHsPeed the SimpleQuery is created dynamically to allow direct access to security tables, audit trail. Using the DBTable or DBQuery would add a lot of code that is not required here.
The simpleSecurity module is kind-of the same story. It is used in our Restful services where you don't have menus to maintain. It also allows you to integrate with external logon tables of CMS etc. The full RBA module is aware of all the menus and components and will control access to them. A lot of overkill in a web service or tables of different origin.