Is there any good reason to disallow scriptlet or EL expression to be inserted as attribute value?

Let's say we have tag:

<tag>
    <name>mytag</name>
    <tag-class>org.apache.beehive.netui.tags.tree.Tree</tag-class>
    <attribute>
        <name>attr</name>
        <required>false</required>
        <rtexprvalue>false</rtexprvalue>
        <type>boolean</type>
    </attribute>
</tag>

What could be a good reason for dissallowing the below?

<my:mytag attr="${setting}" />