Why will airline crews use a rolling take-off?
Because it is recommended by the (turbofan) engine manufacturers.
When at high powers settings at low (no) airspeed, a huge amount of airmass is sucked into the engine.
While inflight all of this air will come from the front, at static thrust the air will be sucked from the sides of the engine narcelles as well!
This sideways airflow -can- cause a disruption in the airflow entering the main compressor, which -can- lead to an engine stall.
This is the main reason why turbofan engines, when on the testbench and tested at high powersettings, will have a bell shaped narcelle attached to it, instead of it's regular one.
On the plane however, a bell shaped narcelle would be foolish, because the engine is designed to operate
in an environment where air inlet speeds are somewhere in the 150 - 200m/s range . (normal turbofan on e.g. a 747 or similar type of airliner). The drag would be increadible high!
In this type of environment an efficient narcelle is slightly tapered at the inlet, and a little wider near the fan disk.
(sort of an inverted bell shape, but less dramatic).
This will cause the oncoming air to decelerate slightly. The total pressure of this air therefore will rise, which will make
a positive contribution to the massflow entering the turbofan. This is one of the main reasons turbofan engines are highly successfull in generating thrust at high altutudes with a relativly modest fuel flow. Remember, a turbojet / turbofan core will suffer from the decreased airpressure at high altitudes! This is where the engine narcelle will come to the rescue. It can be responsible for up to 30% of the total thrust at altitude. (ram recovery.)
To accomodate for the high pressure setting / low airspeed scenario down the runway at t/o, the engine narcelle has a quite thick and rond edge, which will prevent airflow disruptions. (still, the rolling t/o recommendation will be given)
With the rolling t/o, the runway used penalty will be relativly small. As stated elseware in this forum, initial acceleration is high (somewhere around 2.6m/s^2), because of the huge massflow throught the engines. When the aircraft picks up speed on the runway, engine thrust will become less.
All commercial jets are hugely overpowered, to cope with n-1 scenario's and the FAR requirement to maintain a 2.5% climbpath with full load.
Usually, when atmospheric conditions permit, a derated thrust is used. This means usually there is runway length to spare.
Remember, the official law stipulated a 35ft screen hight. Most aircraft will be well above the 35ft imaginary barrier when overflying
the runway threshold at the other end.