Again:

The correct behavior for an aggregate concatenation query is undefined.

The vendor is not giving any guarantees, and this is the key issue.

Up to SQL Server 6.x GROUP BY implied an ORDER BY. Then Hash Aggregates came along and broke that.

Up to SQL 2000, you could use TOP 100 PERCENT ... ORDER BY in a view definition, and a SELECT from the view was always ordered.

That is, what works today, may not work tomorrow, and Microsoft may just shrug their shoulders.

On the other hand, if FOR XML PATH breaks, you can scream "bug" and they will have to fix it.