Consider the case of 2 paddlers arriving at a 2k portage with a canoe and 2 packs. They walk the packs through first, and this trip takes them 20 minutes. One of them goes back for the canoe which takes 40mins for that round trip and a total time of 1 hour. The other paddler waits around for the 40 minutes.
If they did the trip and a half method they would walk the packs half way in 10 minutes. One would drop the pack and go back for the canoe while the second would complete the portage and then return for the pack. They would both complete the portage in 40mins thus saving 20mins.
In general, if somebody is waiting for a long time at the end of a portage they are doing something wrong.