You're oversimplifying it. If you can guarantee that you're starting from a steady state, then, yes, activities are nothing more than macros. But you can't guarantee that, even with discrete codes.

Example setup: you're watching your DVD (input 1), you then switch to an always-on DVR (input 2), then you listen to CDs.

• Examples without discrete codes:
  • Macro:
    • (1) send TV power signal, guess at input 2 signal
      [TV turned off, DVD player still on, possibly playing, video input may or may not be right]
    • (2) send CD player power signal, send amp power signal, guess at amp input signal
      [TV still on (assuming you manually fixed it in the last step), audio input may or may not be right]
  • Activity:
    • (1) send signal to go from input one to input 2, send power signal to DVD player
      [everything OK]
    • (2) Send power signal to CD player, send power signal to TV, send power signal to amp, send signal to move from last input to CD input
      [everything OK]
• Examples with discrete codes
  • Macro:
    • (1) send TV on signal, send DVR input signal
      [DVD player still on]
    • (2) send amp on signal, send CD player on signal
      [TV still on]
  • Activity:
    • (1) send DVR input signal, send DVD player off signal
      [everything okay]
    • (2) send TV off signal, send amp on signal, send CD input signal, send CD player on signal
      [everything OK]

And that's a really basic example. When you start having to control more things, and selecting separate audio and video inputs, etc., macros become completely useless.

That said, the fact that the Harmony remotes barely have any macro facilities is bad, since there are some things that are impossible to do with activities. Say, for example, turning on closed captioning when it requires that you go through a menu.