I assume that most, or possibly even nearly all travel agents aren't motivated enough to optimize such complex itineraries, factoring in each traveler's preference (eg sensitivity to red eyes) together with ticket prices and routing etc.
Not to mention cross-traveler effects, like preferring a slightly more expensive flight if it allows multiple people to arrive at about the same time and share a taxi...
So I prefer having an assistant who can dedicate much more time and attention... and use something better than a spreadsheet to manage it all.