Lyxen
Great Old One
The problem with this interpretation is that you have basically told 3/4 of the players that they may as well have their characters build Legos or play cards narratively because their decision to help keep watch didn't result in a world where the party was more alert than if they hadn't been doing so.
It might seem so, but it's not really the case. But first, it's not a bad thing, since it also encourages players to do something constructive during their travels, whether it's one of the actions described in the travel rules or something more "roleplaying" like "I don't feel useful with all these guards around, so I'll just go and try to seduce the caravan master", etc.
Second, as mentioned even in the travel rules, the applicability of the people on watch might depend on the circumstances. Some threats might be in front, others in the back for example. It then becomes a question of who wants to be where, and do what. It also comes up if the main "perception" resource wants to do something else, like forage or track. And even if it's a ranger who can do two things at once, if he is foraging or hunting, see applicability just above, it might be better to have another lookout with the party. Or escorting a large caravan, there might be need for people front, back and on each side, etc.
So I have never found this a problem, honestly, although I must also say that I've not used these rules a lot, since we tend to summarise the travel. But when we have used them, the players have enjoyed the repartition of tasks, including who cooks dinner (old joke from Eddings), who does the washing, etc. It can be roleplayed very enjoyably, and when the DM creates encounters that build on the variety of tasks, it's even more enjoyable. And players are happy when their secondary task provides something useful: "you encounter another river, but from the mapping done by Aphitéa, you are pretty sure that it's one that you've seen, only a bit more downstream since it seems to be larger, etc."