A good example of a flight plan that will never be "generated". BA when flying from Gatwick EGKK to Turin LIMF often have to fly via Zurich. For reason of traffic, time of day and French ATC strikes (which are all too frequent!). You could for instance create a fpl just to Zurich and then another to Turin. But even that would be wrong. Firstly the route to Zurich would be too far to the West. The second point and perhaps more important is that there is no direct airway to Turin from Zurich. Which means that a large part of that leg is off airway. This illustrates my point that generated fpls should never be taken as read. In the sim world perhaps some would say that it doesn't matter. But it does in the sense that it is unreal and inaccurate. It takes very little intuition really to think that a specificly generated fpl doesn't look right. Especially if it seems to take you for a tour round the houses.
Always check the fpl and see if it has any strange dog legs especially over significant water (where the rule is direct great circle). It is quite in order and normal to fly between airways ie. off-airway. You would in real life be asked to report crossing various waypoints. This is not yet a feature in PF3 but has been earmarked for consideration.
Key points to look for. Does it follow airways in the correct direction? Is it more or less direct? If not plan to go direct to the next wypt instead. How far away is the last wypt from the destination? You may need to add a wypt or two in order to take you to the relevant STAR. Another point to consider for complete accuracy would be whether the fpl overflies prohibited airspace and/or if there are altitude restrictions. This would be more likely on an approach leg.
Some generated fpls are very good but even so still need checking.
The addition of SIDs and STARs are another matter as they too are frequently wrong especially where more that one SID or STAR leads to the same wypt. The only way out of that is to check the airport charts carefully for local restrictions and then input the correct SID or STAR for that route.There are plenty of simmers who are deeply under the impression that only ATC issue SIDs and STARs. The is most definately not the case. The idea of a SID is to take me from a specific runway to a specific wypt. That wypt is the start of my fpl. Clearance delivery is nothing more than a confirmation that ATC and the crew are singing from the same hymn sheet. That ATC are talking to the "correct" crew. It is quite common to enter a SID as a specific part of a fpl. ATC would have to have good reason for sending you in a totally different direction especially if the flight was fuel critical. Likewise the last wypt on a fpl would be the start of a STAR to a specific runway. Usually you can programme alternative STARs into an FMC. If not PF3 allows you to route by yourself to the changed runway.
So creating a fpl is definately a black art and one reason why I don't spend money on certain programmes as I can easily do it manually and get a better more accurate result into the bargain.