Datatypes and mixed content

Hello,
As I am aware of, defining that the text between the subelements of an
element defined as having mixed content is not allowed.

http://xsd.stylusstudio.com/2001Dec/post01012.htm
"XML Schema doesn't allow you to control the type of the text that you have
in mixed content"

Can anyone give any hints regarding why this is the case, except for the
fact that the standard says so? Any reasons it should not possible to
control the type?

Regards
Trond A Myklebust