Simple Link Inconsistency?

In Chapter 5.2 of the XLink Rec it is stated that "A simple link is a link 
that associates exactly two resources ...".

However, in Chapter 5.4 it is stated that the so-called "locator attribute", 
xlink:href, "may" be used on a simple-type element.

If a simple-type element *must* associate two resources how is it possible to 
make that association in the absence of an xlink:href attribute? - a 
situation that Chapter 5.4 indicates is permitted.

It seems to me that Chapter 5.4 should be amended to indicate that xlink:href 
is required on a simple-type element.

Andrew Watt