EngStudent,
At the design stage, aircraft are Payload / Range optimised for their typical mission profile.
For example, an aircraft designed for Sydney - Los Angeles operations must be able to carry a profitable payload at the same time as carrying sufficient fuel for the flight. If Takeoff and Climb fuel efficiency was less than optimum, then it's still acceptable provided that cruise operations, which will take up about 95% of the flight time, are efficient.
On the other hand, if we could stuff enough fuel into a B737 to operate Sydney - Los Angeles direct, the payload would be nil or negligible. This aircraft has been designed for Short / Medium range operations, and Takeoff / Climb fuel efficiency (where the aircraft may spend up to 40% of the flight time) must be good, even if cruise performance is only mediocre.
There's absolutely no reason why a narrow body aircraft could not be built optimised for long range, or a wide body aircraft optimised for short range, the simple fact is that nobody builds such aircraft. (At the same time, wide body aircraft such as the A330 offer very good short range, as well as long range economics).
To respond to your one uncertainty (is fuel saved on descent), yes, absolutely it is, but fairly typically, the fuel saved on descent is then used for the approach, making little difference to calculating the flight at cruise to overhead the destination.
Regards,
Old Smokey