Hi Crossbleed,
Leaving useless waypoints in the flight plan page (Airbus) can create several interesting issues.
For instance, if the flight plan page is not cleaned up when radar vectors commence the distance to run will be in error as will the calculations for fuel on touchdown.
If the waypoints left in the flight plan page do not sequence properly on approach and you find yourself doing a missed approach it will be without the proper waypoints on the flight plan page and therefore the managed NAV capability may give you some interesting routing, or none at all. At least until you figure out what is wrong and how to fix it :-))
An acceptable procedure is to delete waypoints until a fix on the approach you will fly over is the to waypoint and PPOS is the from waypoint. The FMGC then calculates the distance to go and the fuel based on present position direct to the to waypoint. And it gives you a good navigation picture on the ND.
best regards,
Bruce