Good question and I think that I know the answer.
For best results, the laminations are interleaved (one inserted in one direction and the next in the opposite direction). To reduce cost because transformers are labor intensive, the laminations may be grouped as pairs rather than singly. To reduce labor even further, they may be grouped at up to about five laminations at a time. The drawback is higher leakage flux and higher magnetizing current--not a big issue at power line frequencies.
The ultimate cheapness is to stack all the E laminations together and weld to a bar of all the I laminations stacked together. The drawback is further increased leakage flux, higher magnetizing current, and higher eddy currents that are induced into the core because they are no longer fully insulated from eachother--in other words, efficiency goes down and temperature rise increases. While this is an acceptable manufacturing method, performance suffers.