> To those questions I would add another one:
>   why "if" xpath 2.0 expression demands "else" part?
> 
>   IfExpr    ::=    "if" "(" Expr ")" "then" ExprSingle "else" 
> ExprSingle 

The main reason was to avoid the infamous dangling-else ambiguity:

if (c) then if (d) then e else f

I personally would have preferred the solution of a closing token such as
"end-if" or "fi".

I do remember a half-day spent on if/then/else, where it was clear that
no-one much liked the status-quo syntax, but no-one could come up with
improvements that had majority support. 

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