Reporting Services will only automatically generate a comma-delimited string for multi-value parameters; it won't generate XML. So I would have to write a parser in Reporting Services to express the parameters in XML before passing them to T-SQL where I then have to break the XML down again.

Neither of those is actually difficult, but I don't see the value in spending the extra time every time I call a stored procedure from SQL Server Reporting Services to ensure that any muti-value parameters are encoded in XML.

In this case, it would make the interface between reporting services and SQL Server harder to use or debug.