That is a negative way at looking at it.
You could have one guy doing the roofing job, and it take him 4 days.
Or you could have 2 guys doing the same roofing job, and it take 2 days.
Or you could have 4 guys doing the same job, and it be done in a day.
One guy stripping the old roof, one guy putting down the underlay, one guy doing the flashing in the valleys and one guy nailing down the shingles.
Taking one day to do the job minimizes the time the plywood is exposed to the elements and preventing water leaks.
Craig, I completely agree with you that 1-2 workers on a job site is not a good idea, both for efficiency and safety reasons. In fact, 3-5 to workers is pretty ideal for a typical Vancouver home, but it would be pretty unlikely that they could blow off, clean up and replace an entire roof in a day. A responsible company will remove and replace a roof in sections, minimizing the exposure and rarely relying on tarps. If you hire a legitimate company that know what they're doing, the risk of a leak during a roof replacement is close to zero.
The problems I saw when I worked in the industry were with the crews of 10+ that were actually ripping off and replacing a roof in one day. Protection was terrible, there was always damage to adjacent properties, installation quality was atrocious. Just because it's an unregulated industry, does not mean that all installers are equal.
The point I was trying to make is, like most things, quality generally rises when you take the time to do a job right. That, and you get what you pay for…