Forgetting to cancel does happen more often in Canada than in the USA, because in the USA the pilot who forgets is on the hook for the cost of any search-and-rescue efforts that might be mounted. Whenever we have American pilots flying into any of our airports (our specialty covers both American and Canadian airspace and there's quite a lot of border-crossing) we never have to worry about them cancelling, and they'll often do so in the air if at all practical, especially if they know we're trying to free up the airspace to get a departure off the ground. In Canada, the cops/military don't go after the pilots who forget. As well, unfortunately, at most of the airports I work, we don't have a peripheral frequency or an FSS on the field to try to talk to the aircraft when they're on the ground. The majority of our cancellations get relayed through a 1-888 number that the pilot has to call after landing, and as you can imagine it's not that uncommon for somebody to forget to grab the cellphone or duck into the FBO after landing and going through all the shutdown stuff.
Under Canadian rules, we have to shut down the airport completely for half an hour after the aircraft's ETA. By that time we'll have the cops on their way to go look for the aircraft. After half an hour, we can start clearing approaches in again, as long as we tell the pilots about what's going on and advise them to be on the lookout for the concerned aircraft, since there's a possibility (albeit infinitesimal) that he still might be flogging around somewhere in the vicinity of the field.
Previous posters are correct that this happens mainly at uncontrolled fields where the "one-in-one-out" rule applies (in fact, many of the airports in our specialty are so close together that clearing an approach into one of them effectively shuts down any other approaches OR departures at four or five separate fields). However, it's not entirely correct to say that a cancellation from an inbound into a controlled field wouldn't create any operational advantage for us. For instance, the rule at the one towered field in my airspace is that once an IFR inbound has passed a certain fix which is about 8 miles from touchdown, we can't get a departure off. Ensuring this separation is actually the responsibility of the radar controller (me), NOT the tower controller as this tower isn't certified to provide IFR separation outside the control zone. What happens is that the tower will call me on the hotline and say "ABC123 is ready to go at runway 30" and if I have an inbound who's more than 8 miles out I'll say "ABC123 valid off runway 30, [departure instructions], cancelled when XYZ234 reports FIXXX." Then I'll clear XYZ234 for the approach and switch him to tower with the instruction to report FIXXX on tower frequency. If that happens before the departure goes, then the clearance I gave is no longer valid and tower has to hold ABC123 until after XYZ lands. In that case, tower will call me back on the hotline and I'll change the clearance to "ABC123 valid off runway 30, [departure instructions] *when* XYZ234 is on, over, or cancelled." On = landed, over = tower assumes control (he can provide separation within his 5-mile control zone, but he can't do the actual approach sequencing), cancelled = cancelled IFR.
So if XYZ234 just happened to cancel IFR when he was 6 miles out, we no longer have to provide IFR separation for him, so as long as we can get ABC123 off before he touches down, we're good to go. It doesn't happen very often, but it can happen -- especially when the departure that's waiting and the arrival that's causing the delay happen to be from the same airline (and we're always quick to let them know they're holding for company traffic inbound -- sometimes, at an uncontrolled field where the wait for an aircraft to shoot a full-procedure approach can be quite long, the two aircraft concerned will go over to company frequency and work something out, then come back to you and let you know what they want to do). And that's why a pilot might choose to cancel IFR in the air even when landing at a controlled airport -- long-winded explanation, but hopefully it helps.