xs:assert and Schematron

I have tested xs:assert in the working draft of XML Schema 1.1 as
implemented in Saxon.

It surprises me that we now can do anything in XML Schema directly,
that we can do with Schematron. Except for Schematron's user-defined
error messages and a nice report format.

So this is my question. Is there any testing we can do with Schematron
that we can't do in XML Schema 1.1 directly?

If we also get user-defined error messages in XML Schema 1.1, for the
xs:assert element something like:

<xs:assert test="some checksum validation of  ISBN number">
	<xs:annotation>
 		<xs:appinfo>
			<xs:error>Book {./title} has an invalid ISBN number
{./@isbn}</xs:error>
		 </xs:appinfo>
	</xs:annotation>
</xs:assert>

and even the same facilities for other error conditions, such as not
satisfying a pattern or an enumeration, as suggested by Michael Kay in
https://sourceforge.net/forum/message.php?msg_id=5819277

what would then be the raisons d'être for Schematron except for a nice
validation report format and as an alternative to cumbersome XML
Schema in some situations?

Cheers,
Jesper Tverskov
http://www.xmlplease.com