I know it was a great scene and established where the Dr. came from but it wasn't really necessary. If you take that scene out, Bane's first reveal is in the sewer. Was the opening plane scene an added action sequence to start the movie with a bang rather than a speech about Dent?
MAJOR SPOILERS AHEAD
Imagine it's a week before the opening scene. Bane is scheming on how to destroy Gotham, learns of Wayne's fusion reactor/neutron bomb via Miranda/Talia, and decides it will be his main device to hold Gotham in fear while he lets them destroy themselves from the inside.
Bane also comes to learn Dr. Pavel is the only living person who can defuse this bomb. He feels it will be easier to capture Pavel earlier than later; that way no one will be able to call on Pavel in Gotham's upcoming dire moment of need. (It's likely at this point that Bane already had some plan to fake Pavel's death, albeit in a less grandiose way than what he ends up having to do.)
But wait. Uh-oh. The CIA is looking for Pavel too! Bane and his men quickly adapt to this new information. Little does the CIA know that the man bringing Dr. Pavel in is actually Bane's top lieutenant, injected into the operation by Bane when he realized the CIA might capture Pavel first, severely diminishing the chances for his operation's success.
Now we're at the opening scene. Bane and his militia don hoods and fake their own capture, and Bane's lieutenant hands off Pavel to the CIA. In this dialogue, we hear Bane's lieutenant explain, "They were trying to rack a price [on Pavel]. They worked for the Masked Man [Bane]." The CIA agent, being the smart cookie he is, realizes he has an opportunity to track down a lead on Bane, probably hoping to eventually capture him. Little does he know that the hooded men are actually Bane and his militia, who have anticipated his wanting to board them on the plane after hearing such a juicy tidbit.
This is where things start to go sour for our poor CIA agent.
Some more of Bane's militia now take control of the CIA's plane from the outside, necessary for two reasons:
- Bane and his men would not have been able to get weapons on-board otherwise.
- Bane still needs the rest of the world to not be looking for Pavel. He still needs to fake Pavel's death. Thus, he brings a recently-deceased body on board and transfuses some amount of Dr. Pavel's blood into the dead body.
Bane now breathes a huge sigh of relief. He can rest easy knowing that when the plane goes down, a body containing Dr. Pavel's blood signature will go down with it, thus tricking whatever poor sap has to investigate the plane wreckage into thinking Pavel went down with the plane.
The only remaining task: make sure Pavel stays alive. Bane grabs a hold of Pavel, fastens him securely to the rope extending from Bane's plane, and, just before releasing the CIA's plane from any further duty, assuages Pavel's concerns:
Calm down doctor, now is not the time for fear. That comes later.
tl;dr: This scene was necessary because it foreshadowed later events.
Also, it brings up the theme of intentionally being captured. Bane was intentionally being captured so as to have access to Dr. Pavel. Later on in the third act, Bruce gets intentionally captured so as to have Lucious Fox access R&D, and then Selina Kyle saves him.
In 'The Dark Knight' the Joker is also intentionally captured so he can help Lau escape and access all the mob's money. Perhaps a coincidence, but as we just learned, we're not allowed to believe in coincidences anymore.
Notably the plane scene was screened as a special prologue "before Mission Impossible: Ghost Protocol on only 70mm IMAX theaters." (source)
Perhaps the filmmakers had the plane scene in mind for somewhere more toward the middle of the movie (where I think it would seem to fit more neatly) but then found a way to make it work as the opening scene so they could release it as the prologue mentioned above.