Form and function help set one in the proper frame of mind, and aids in memory recall and appropriate behavior. Likewise, it is easier to remember where you put something in a small bedroom, than in a large room that serves many functions. Taking this analogy back into the world of computer environments, it should be the case that once the initial cost of learning how to operate in a given environment is overcome, say learning to use a new program, people would be more productive if their environment was conducive to the task at hand. The total amount of time using any program should be far greater than the amount of time learning how it works. If the interface is conducive to using a program, learning a new, different, but more efficient way, ultimately provides the more user-friendly experience.
Uniformity makes differentiation between options much harder.
In my experience talking to people, once they get over the fear of trying something new, and start learning how to actually use a computer they are not only happier using it, but enjoy learning more. Earlier today, I spoke to a secretary, who although near to retirement, recently learned to write her own database and is developing an interface to meet the needs of the people in her department. She admits to having been technophobic, and did not start using computers until age 51. However in the past year, she has finally broken out of the 'cult of the average user'. Because she read the manual and learned how to actually use the program, and not continue to simply get by using only that which was initially familiar.