OP_CAT Labs' Liu says Satoshi envisioned Bitcoin to be programmable. To get there, one piece of code needs to be re-enabled.

Since there are basically no side effects to enabling an Opcode cache, it’s recommended that all production sites utilize one. On larger PHP applications like WordPress or Drupal, performance ...

Searching for holes in the opcode table. Some of the undocumented instructions are probably not useful. They are either impractical or duplicate a function you can already do another way.