A few years back when direct tracking and requests therefore came up for discussion the major airlines of the day said that as a matter of policy they plan all their operations on established tracks and flying the full SID or STAR as applicable and do NOT encourage their pilots to seek direct tracking in any form.
ASA was of the belief that safety assurance would be maximised especially in terminal areas if direct tracking was not provided - the everything on rails policy.. The airlines stopped short of instructing their pilots not to seek direct tracking but from an official position they did not seek same.... leaving it up to the pilot.
The best person in the system to judge if direct tracking or any other tracking is best for an aircraft is the ATCO or Flow controller and if it does not conflict and serves to expedite traffic flow in a safe manner then perhaps it is the ATCO that should "offer" the direct tracking in every case. That would mean that if is not available it would not be offered, and of course if it worked right the pilot would never have to ask. (if the pilots trusted ATC to always offer when available that is..)
But that is not the culture that our system works on as the communication at a high level seems to not have worked as it should have.
Perhaps "amended route" is the way to go...?