[dvwjr]

The reason a terrorist wishes to explode his assembled device at 30,000 feet instead of at the terminal when landed is that a small bomb assembled by a team in flight does a lot more damage than a much larger bomb exploded on the ground. Not that much fuel in most aircraft when they land. There is a certain logic to it...

If you use a 'larger' bomb on the ground, the folks on the aircraft who are not killed by the explosion may attempt to debark to the relative close ground. A much smaller bomb exploded at the aforementioned 30,000 ft in a pressurized coke-can which is the aircraft fuselage gets additional help in destroying the aircraft skin by the internal pressure and the force of the 550 mph air going past the aircraft's now ruptured skin. A another happy benefit for the terrorist is that those who are not killed by the immediate explosion of his bomb will not probably survive the lack of oxygen at 30,000 ft, nor the fall to the ground below.

Your terminal idea could work, but why put a team on the aircraft when you could have a team on the fuel truck set to explode, plus your ground team has a good chance to escaped. In the air, short of a 'James Bond'-like parachute escape, the air team buys it...

dvwjr

[VOA]

A much smaller bomb exploded at the aforementioned 30,000 ft in a pressurized
coke-can which is the aircraft fuselage gets additional help in destroying the aircraft
skin by the internal pressure and the force of the 550 mph air going past the
aircraft's now ruptured skin.

It takes a disturbingly small amount of explosive to blow a very large (fatal) hole
in a commercial jetliner...
as evidenced by the photo of such a test on a pressurized airliner posted
here in the wake of 9-11.

[timpad]

...a small bomb assembled by a team in flight does a lot more damage...

All good points and I would think the standard reasoning for those who would do such a thing. The idea I put forward in my post was a response to the unusual nature of the activity reported in the story.

• A 14-man team seems large to assemble a small bomb.
• The last trips to the bathroom began after the flight attendants had already secured the cabin for landing. Blowing up the plane at cruise does not seem to be the goal. 
• Destruction in flight fails to accomplish two of the terrorist goals, mass casualties and theater. (excluding the airborne agent dispersal concept)

But sitting here thinking, another scenario comes to mind that blends both. Destroy the aircraft in flight, but at low altitude over the city. Mass casualties, destruction, economic impact, etc.