I don't know about other people, but I have two reasons that seem pretty good to me.

1) I share my wi-fi with my downstairs neighbor, and it would not be fair to impose the scheduling of my internet usage on her.

2) My telephone service is VOIP. No router, no phone.

Even though my computer is off when I'm not sitting at the keyboard, the rest of the network (printer, neighbor, SWMBO's Macintosh [if she's using it], telephone) remain on.

tanstaafl.