oicur12
It boils down to maintaining control rather just monitoring and hoping that that everyone does the right thing.
As good as modern FMS are, I can tell you for a fact that everyone does not follow the published ground track as accurately and in the same way as you might like to think. I can't give you a techinical explanation but there are obvious differences to us on radar in the way different types of FMS interpret and (try to) achieve the desired flight path which can also depend to a large extent on how the coding suppliers have interpreted the published procedure and coded the path terminators to the system.
So rather than sweating on whether somone will turn base or onto the final intercept at exactly the right moment to achieve the spacing we are after, controllers would rather control and ensure the right result.
For info, losing just 0.2 NM between each aircraft on final loses us about 2 arrivals per hour. Somethng that the Airlines and AAHK are never happy about.
Hope that finally answers your question.