You don't know when to preempt - but that's what the timer is for. It would be something of an emergency measure, because most of the time, most threads go to sleep for some other reason than running out of time. (Of course, if a thread regularly exceeds its time slice, the computer won't go into an idle state - but that's the right thing for it to do, because it isn't idle.)

As for checking for something happening, do you need to poll for this? I don't think it's necessary in the general case. Perhaps there is some broken hardware that doesn't support interrupts, but I have a hard time believing that's very common. Aside from that, nothing ever happens except by request, and (if structured right - not that I'm suggesting this is automatically easy) polling is unnecessary.