I would guarantee the last thing he is thinking about is an airport's flow rate
I agree that he OR SHE does not need to consider flow rates, I was just highlighting it as a reason why operators would certainly endorse the use of intersection departures at Luton, assuming the conditions were appropriate.
We will nearly always calculate performance for an intersection departure. Exceptions are during LVP's, if windsheer has been reported, a contaminated runway, anti skid inop, thrust reverser inop etc. If the weight is close to the limits for an intersection departure we'd probably plan on the full length, but have the figures ready for an intersection in case the tower ask us if we are able to accept the intersection for a quicker departure. There are often occasions when the tower can comfortably get you away from the intersection but do not have the time to allow you to back track. There are also occasions when the aircraft No 1 at the hold is on a Compton departure and the tower is unable to get a release from London due to a Northholt departure. In these cases the CPT departure can back track and await the release and Olney, Dover or Clacton departures can go from the intersection. This can save you a lot of delay.
When we taxy out we tell ground if we can accept the intersection and it helps the tower with their planning. On some SID's you need 2 minutes behind the proceeding aircraft so there is no advantage in taking the intersection, in these cases 99% of pilots will back track and take the full length. When the tower instructs you to line up they will either say "line up at the intersection" or they will say "line up runway 26, back track available". PAXboy, I suspect your crew were told that there would be a short delay for their departure so they sensibly chose to back track.
By the way I saw a Wizzair take the intersection yesterday so clearly they are allowed to do it.
SW
PS. I've never used the full runway length at CDG