> ...which is a bit odd to me at the moment.  It seems a bit of 
> an artificial restriction.
> 

The basis for the distinction is the theory that with mixed content, it
makes sense to treat the markup as mere decoration of the text, whereas with
element-only content, concatenating the children is a meaningless operation.
In both cases I think the assumption is probably correct 95% of the time,
and in the 5% of cases where it's wrong, the rule doesn't do much harm.

Michael Kay
http://www.saxonica.com/