Leaving aside the issue of direct routings, which should be solved by more and more flight plan based direct routings, the fact remains that a large percentage of traffic is requesting a different level than the one they have in their flight plan. If we could have companies filing what they actually want and if those companies would ensure that their crews stick to these plans, we would make a huge step forward.
A margin for operational reasons, from both an ATC and a flight deck perspective, should always be included. It is not the idea to put pilots and controllers in a straightjacket.