[Aetius]

  The software entity is constantly subject to pressures for  
  change. Of course, so are buildings, cars, computers. But 
  manufactured things are infrequently changed after 
  manufacture; they are superseded by later models, or 
  essential changes are incorporated into
  copies of the same basic design. Callbacks 
  of automobiles are ready quite infrequent; field changes of 
  computers somewhat less so. Both are much less frequent than 
  modifications to fielded software.

  In part, this is so because the software of a system 
  embodies its function, and the function is the part that 
  most feels the pressures of change. In part it is because 
  software can be changed more easily--it is pure thought- 
  stuff, infinitely malleable.

- Fred Brooks, No Silver Bullet

[billswift]

Good essay but I don't think he adequately addressed the problem of transaction costs for changing software, either in process or later updating, of course that wasn't his primary point (it has been a while since I read it and I could be misremembering).