Short answer:
Operational efficiency - significantly, you have saved time, and you might also save fuel from an unnecessary longer taxi.
Long Answer:
1. Operational Efficiency:
As discussed above, taking off from an intersection saves taxiing time. If you save this on two occasions during the day, you have saved 10 minutes. This might even mean the difference between missing an ATC slot restriction on your third sector later in the day. Remember, many major short-haul carriers seek to turn around their aircraft in less than half an hour. Minutes matter.
2. The takeoff is still completely safe from the intersection:
The aircraft in your question took off from the D intersection at LFSB/BSL.
From Intersection D: 2900m/9514 ft Takeoff run available (TORA)
Full RWY 33: 3900m/12795ft TORA
Here are some performance calculations for a 737-800 (reasonably analogous to the aircraft in your question), loaded with (a typical sort of flight) - 7 tonnes of fuel, 175 adult pax on board, and 700kg of baggage in Hold 2. The performance figures are from the approved source for LFSB/BSL: wind calm, 15 degrees Celsius outside air temperature, dry runway, and 1013 hPa QNH.
- Intersection D: De-rate to 24k, using assumed temperature thrust reduction (known as Flex for other types) of 41 degrees C, v1 140 kts, Vr 141 kts, V2, 145kts
- Full RWY 33: De-rate to 24k, using assumed temperature reduction (known as Flex for other types) of 41 degrees C, v1 140 kts, Vr 141 kts, V2, 145kts
From the data above, you can see that the aircraft will be set up identically for takeoff from runway intersection D or if it were to take off from the full runway.
So, in this instance, the further taxi from intersection D to the full runway would be around 1km (cf. airport charts) or perhaps around 120kg of fuel burnt. This may not sound like much, but if that airline operated 2000 flights per day, then it is a saving of almost 250 tonnes of fuel - costing perhaps around 200,000 $US, in just 1 day.
Please note that there may be circumstances where higher (or "less" de-rated) performance is required when the takeoff run is shorter from an intersection, which would potentially lead to higher fuel burn on the takeoff roll or more wear on the engines.
3. Takeoff safety for jet aircraft is arguably less about runway distance available and more about critical decision making around speeds close to V1.
Taking off from intersection D gives you a takeoff run available (TORA) of 2900m. To put that into perspective, the TORA at Bergamo runway full length is 2874m, TORA in Faro full length is 2490m, TORA at London Luton, both runways full length is 2162m (and on a hill), or finally at Bristol, where the TORA is 2011m (and on a hill). All these airports have a TORA less than that available from the D intersection at LFSB/BSL and are frequently used by jet aircraft without any concerns over safety.
At v1 speed of 140 knots, a 737 would travel the distance between the D intersection and the full runway length (1000m) in 13.89 seconds. (And 1000m is a large difference in length between intersections). Perhaps a more average distance between intersections is 400m, which gives crew another 5.56 seconds of decision time (really not much)…
Arguably what is therefore more critical from a safety perspective is not necessarily the increased runway available, but rather the decision making skills of the flying crew. If the aircraft was unsafe to takeoff the crew need to decisely act before v1 to reject the takeoff.
Furthermore, if the worst eventually happens (aside from a dual engine loss at v1), such as a major engine damage or fire at v1, the aircraft is still safe to fly - and flying crews regularly train for this scenario. It can actually be safer to get the aircraft into the air, secure the engine and return as opposed to stopping or trying to immediately land it back onto the runway.
As an airline pilot working for a major European airline I can testify that we regularly aim/plan to take off from intersections. We run the performance figures for intersections and will typically request from ATC to use an intersection if the cabin is secure for takeoff. (It is often the lack of cabin secure which prevents us from taking a planned intersection - cabin not ready so we “might as well continue” to the next intersection/runway endpoint).
Your question asks,
Wouldn't it make sense…to have more runway available
Safety is priority, however operational efficiency is also a priority.
The takeoff is safe from the intersection: now we think about saving time and fuel.