From what I have read on this thread it seems that some companies have got this Discretion thing wrong, big time!
Under the licence that I am flying on:
1} No Capt can dispatch from Base if he knows that the flight will require discretion to complete. So we can't leave A to go to B if we know that B to A will need discretion.
2} Discretion can only be used if in the Capts opinion the company has made every effort to avoid its use. This means that it really is an unavoidable delay down route.
3} It is the Captains discretion to extend a duty and no one elses! I for many reasons I have elected not to use discretion many times and never been invited for tea and bickies!! Our licencing authority would love to hear from a Capt who had been questioned as to why he refused to go into discretion!
4} ANY use of discretion must be reported to the licencing authority.
These rules are respected by the company and they will either put on a heavy crew or position crew to avoid problems.