on zero or any repeat

Dear all:
   I have one question on XPath specification.
   In Xpath, the operater * denotes the wildcards of XML and // denotes the ansester-
descendent relationship of XML element. So I wonder why XPath could not support the 
zero or any repeat of sub xpath, such as /(a/b)*/c, which means we can pass /(a/b)/c
or /a/b/a/b/c ... in XML data document. I know such expression could be replaced by
//a/b/c, but the former expression seems be evaluated efficiently.
   I know such expression can not supported in the current version of XPath. I only want 
to know why such an important feature can not support in XPath, and whether it maybe supported in the future version.
   Best regards
   Thanks
Josh