Questions related to XPath 2.0

I had 3 questions related to XPath 2.0

1. What is the rationale behind including for expressions, conditional
expressions etc in XPath 2.0? From a glance, I would have preferred if
XPath only allowed us to specify patterns, and XPath engine will return
all the occurrences of the pattern (say in document order etc).. I might
have preferred if these things were in XQuery/XSLT and not in XPath..

But I believe there should have been a rationale for this? Can you
summarize/point me to that?

2. I was looking at fn:id (string) and fn:idref (string). How do I use it,
say if I get a list of values of idref attributes. An example that I may
think of is a schema (rough DTD) like:

<!ATTLIST Professor (pname, office, pid)>
<!ATTLIST Student (sname, major, advisor)>

advisor is an IDREF attribute, pid is ID attribute.

I want to do something like:
Student [@major='CS']/@advisor->Professor/@pname

Basically names of advisors of CS majors. How do I write in XPath, can I
do it w/o using ForExpr

3. Another thing I noted is: When I look up fn:id, the spec does not
mention whether the result will contain duplicate nodes. I see that the
spec mentions for fn:idref that the result will not contain duplicate
nodes. I hope this was just overlooked?

thanks, and best,
murali.