Limited complexity requirement?

While thinking about xquery-based designs, I realized I have been 
assuming the following requirement. What do other folks think?

3.X Limited complexity

Less expressive languages are easier to implement, deploy, secure, and 
optimize (cf the Principle of Least Poser in an essay on design 
principles for the Web[1]). Since a large and interesting class of 
applications can be addressed with query languages that are less 
expressive than programming languages, this design should not involve a 
turning-complete query evaluator. The halting problem must not be 
expressible in this query language design.

[1] http://www.w3.org/DesignIssues/Principles

--
Dan Connolly, W3C http://www.w3.org/People/Connolly/
mobile: tel:+1-816-616-6576