Many moons ago, before the double-run concept came about, competitors were guaranteed to have 2 Regular, 1 Jumpers, and 1 Chances each day (save for games-only trials, at least). This meant that regardless of how many days of a trial you entered, you'd at least have a shot at your core NATCH classes. Now that the double run format is so popular, it's pretty common (at least in my area) to see a schedule similar to:
Friday: Regular (2), Chances (1), Jumpers (1), random games
Saturday: Regular (2), Chances (2), random games
Sunday: Regular (2), Jumpers (2), random games
If I was only able to attend the trial on Friday and Saturday, I would only have one shot at a Jumpers Q. If I consistently am unable to show on a Sunday then I will quickly become deficient in Jumpers Qs just based on limited availability. The swap option is appreciated because it lets people enter the runs they need.
I get that there is a lot of flexibility now, but it is still unlikely that trials are going to be able to offer classes that will make everyone happy when using the double run format. For example, a recent Facebook post asking what class people thought was the most fun resulted in one club proclaiming that they would offer Tunnelers 6 times in a weekend. Some people might think that's the most awesome thing ever, but I see that and go, "ugh, I'll pass."
I've never liked the double run format and this sort of thing is just one more negative against it. I wish Regular, Jumpers, and Chances still had to be offered every day of a trial.