why the output requirements on viewport?

Why does the XProc spec require viewports to have a primary output? 
http://www.w3.org/TR/xproc/#err.S0006
>
> The p:viewport must contain a single, /primary output port 
> <http://www.w3.org/TR/xproc/#dt-primary-output-port>/ declared 
> explicitly or supplied by default 
> <http://www.w3.org/TR/xproc/#primary-input-output>. If that port has 
> no /connection <http://www.w3.org/TR/xproc/#dt-connection>/, then it 
> is connected to the /primary output port 
> <http://www.w3.org/TR/xproc/#dt-primary-output-port>/ of the /last 
> step <http://www.w3.org/TR/xproc/#dt-last-step>/ in the /subpipeline 
> <http://www.w3.org/TR/xproc/#dt-subpipeline>/. It is a /static error 
> <http://www.w3.org/TR/xproc/#dt-static-error>/ (|err:XS0006| 
> <http://www.w3.org/TR/xproc/#err.S0006>) if the primary output port is 
> unconnected and the /last step 
> <http://www.w3.org/TR/xproc/#dt-last-step>/ in the subpipeline does 
> not have a primary output port.
>
http://www.w3.org/TR/xproc/#p.viewport

I can see that it is generally useful to provide an output (when using 
the viewport as a way of transforming a document by transforming chunks 
of it) but equally it's sometimes unnecessary (when using the viewport 
merely to break the document into chunks for independent processing in 
their own right). It seems to me that in those cases where a viewport's 
sub-pipeline merely consumes the sequence of documents without producing 
any output, the viewport could simply remove the matched elements (i.e. 
it would behave like p:delete, seen from the outside).
-- 

Conal Tuohy
eResearch Business Analyst
Victorian eResearch Strategic Initiative
+61-466324297