I think Steve's point that if they are made too powerful, there is a
real chance that they will be used exclusively is a very real danger
(or to be precise, the danger I see is that people will start shipping
the embedded stylesheet and the document together, thereby losing a
great amount of flexibility (unless one can turn PI processing off)).
Steve (as usual) has a good point about the location requirement. I
too think they should (must even) occur *after* the start tag. While
for HTML this is not critical, for more structured documents, it will
be necessary.
So far, everyone seems to agree that they are of practical value.
TROFF (groff :-)) anyone?