Consider a 300 feet long wall that is freshly cast. Over time, that wall will experience volume change along its length due to temperature change, shrinkage and possibly creep (for P/T). If you run the numbers, you'll get something like 2" length change over the course of time just from shrinkage. Under an imaginary no restraint condition (or under a P/T systems that overcomes silly restraints like column stiffnesses), the wall could slip towards its center of mass and not form any cracks. Or, a more likely situation where a foundation wall is restrained by a bearing surface, the wall will break at semi-regular distances. If you provide weak planes (construction, contraction or expansion joints) in the wall at 20 ft (or less) intervals, you'll have a 1/16th to an 1/8th separation at each joint that can easily be sealed and looks pretty. If you only want to cast 20 ft (or less) of wall at a time, then you are good to go. If the contractor wants to make money though, you will need to give him a detail to create a weak plane under a continuous cast operation (contraction joint).
For a free standing wall, it is often appropriate to match an expansion joint in the building sitting on it. For a below grade wall cast against earth, it is probably more appropriate to consider the foundation wall as monolithic and provide for the buildings expansion as starting from the top of the wall. Some reasons - waterproofing expansion joints below grade is a bit more difficult. Contraction but not expansion (assuming a fully below grade wall) may occur due to long term settlement of surrounding soils causing the joint to slowly close over time and not match the superstructures behavior.
If you have a wall that will not be exposed to site and is well waterproofed on the exterior, I say let it crack and feel free to ignore the shrinkage cracks - let everyone know its going to happen though.
