Section 11.5.3.1 Invoking a Function - http://docs.oasis-open.org/odata/odata/v4.01/csprd03/part1-protocol/odata-v4.01-csprd03-part1-protocol.html#sec_Functions - states

Function imports MUST NOT be used inside either the $filter or $orderby system query options.

This somewhat defeats the purpose of $root, because it might be helpful to

GET Stuff?$filter=ResponsibleUser eq $root/Me/Username

where Me is a function import returning a complex value - function import instead of singleton because it is read-only.