Appender class substitution is not implemented - is there a reason?

[lure]

A short snippet

    <appender name="STDOUT" class="${APPENDER_CLASS:-ch.qos.logback.core.ConsoleAppender}">
        <layout class="${LAYOUT_CLASS:-some.custom.Layout}">
            <hostName>${HOSTNAME}</hostName>
        </layout>

In this example, the layout class may be specified via env variable, which is useful to switch between different receivers.
However, the APPENDER_CLASS will not be substituted, throwing an error:

Caused by: java.lang.ClassNotFoundException: ${APPENDER_CLASS:-ch.qos.logback.core.ConsoleAppender}

Is there any particular reason for this behaviour?
It is tempting to "fix" it by changing line #23 to

appenderModel.setClassName(interpretationContext(attributes.getValue(CLASS_ATTRIBUTE)));

However, it doesn't seem to be a correct fix, as SaxEventInterpretationContext interpretationContext seems to not be used at all