Moore's law says that CPUs improve faster than I/O. Therefore any program that is I/O bound today is likely to be I/O bound for every future generation of hardware. And programs that are not I/O bound today, may become so down the road.

Incidentally on the final point, there was an interesting test that was run many years ago. Multiple teams were given the same spec, but were each told to optimize for a different characteristics (speed of development, speed of execution, maintainability, memory use, etc). Most of the teams managed to come in #1 on what they were trying to optimize for. The team that optimized for maintainability came in #2 on most other characteristics. The team that optimized for speed of execution came in last on most other characteristics.

The lesson from this is that a consistent focus on maintainable code results in more of everything else you need. Yes, there really are times that you're writing throw-away code and can forget all that. But by default, code well and let the other details take care of themselves.