Consider this hypothetical situation and three possible scenarios that can develop from it.
You realize at 6:00 that something you really want to see is on at 9:00 but are afraid you might get distracted (like, if 60 Minutes starts late after a game and the whole schedule gets off of the usual starting times).
Scenario #1: You try to remember to watch the clock as the evening progresses, but suddently you realize it's 9:25 and you missed the first half of the show. Bummer.
Scenario #2: You have a system that allows you to go into your guide and mark the show. Shortly before 9:00, the system tells you that it's about to change to the channel the show is on, unless you take action to defeat the change. Error averted.
Scenario #3: You have the HR20. You have already set it to record the 9:00 show. You suddenly realize it's 9:25. You have several options. You can switch from watching what is on currently and begin playing back the 9:00 show from its beginning. You'll probably finish the show only slightly behind the usual ending time because you'll be able to FF over commercials. Alteratively, you can finish what's on now, knowing that the 9:00 show is preserved for later viewing at your convenience, whether that's at 10:00, tomorrow, or next week some time. You have options.
Scenario 2 gets the job done. You will see your show on time. But scenario 3 also gets the job done. You'll either see the show pretty much on time (as soon as you realize it's past 9:00) or at some more convenient time. You won't be forced to choose between the currently viewed programming (which must be pretty good if you've totally lost track of the time) and the show you'd planned on watching at 9:00.
Scenarios 2 and 3 are different solutions, but they both work and the latter actually gives you more control and more choices. Switching from the familiar Option #2 to the unfamiliar Option #3 will take a small adjustment to your habits but I think it's reasonable to assume that its benefits will make the adjustment worth the small effort.