It also can be used if I "complete" the SID before the end. For example if the SID ceiling is FL140 and I attain that altitude before the end of the SID (which is more often than not nowadays as the ceiling is usually a minimum requireiment). Supposing some of the published SID route is still to be flown then having reached the SID ceiling ATC can clear me to continue my climb and you can, subject to restrictions such as not to overfly a built-up area can request "direct to" even though you are technically still flying the SID. Except that once above the SID ceiling the SID no longer applies other than the reason I have stated.
Where "Resume own navigation" in PF3 is definately not correct is at the end of a STAR when it gives that call in order to let me fly to the ILS.
There are and always will be "grey" areas due to the limitations of the voice set. Fortunately in the majority of cases all that needs to be done is to eliminate some ATC calls altogether and that is what we are looking at and discussing in the beta.
The reason for SIDs are not to reach an altitude safely, but to increase Controllers performance under busy areas. The SID increases dramatically the capability to control more traffic by one controller. For that reason you have to fly the SID, if they give you a shortcut, it only means they are not busy enough which is good for both
The altitude constraint is just one of the many restrictions of a SID. Having said that ATC will clear you to climb to your cruise or step at the point you reach the constraint or they will keep you there for a while. The SID ceiling is just that! Above it there is no SID in effect. Which is why you can either continue or ask for a Direct or ask for a deviation due to weather. As for the magnitude of traffic bear in mind that the sim AI is rarely if ever at the same level as in real life. Think about London Heathrow's operating capacity. I would have to have so much AI running that I would probably end up with 1 frame per week!
So PF3's "release" as it were represents a situation of not a heavy amount of traffic. In any case you can decide to continue flying the SID or ask for a direct. Both cases are realistic. PF3 gives you both options as it were.
"... The SID ceiling is just that! Above it there is no SID in effect. Which is why you .."
Yes you are right I agree with the first part, of course Obstacle Clearance is the main goal, based on the TERPS obstacle clearance assessment is basically de ODP part of the DP, so far so good, but this is not the only reason for a SID, there are two more reasons which are Flow Management and Noise Abatement procedures. These or any of theses two may be the only reason for designing a DP (SID). Reaching the obstacle clearance assessment altitude does not mean there is no SID in effect above it as you say. You still might have to fly the SID for NA reasons or traffic flow management. Unless ... they accept you a shortcut. Well you always can try, but as I said, if they are busy, the chances get low. A good example is Boston Logan, ODP assessment is very easy and quick, in other words, you have just the city and the sea, but most DPs there are designed for Nose Abatement so you have to comply no matter your altitude and if they get really busy, the just forget it. All you will hear is climb via SID! Of course if you reach certain level NA may not be a factor anymore but don't forget flow management.