I don't understand this either, but a lot of pax will add hours to their trip to save a few $ or to chase status. Just read a few of FT threads and you willsee this craziness here a lot.
As someone who does this regularly I don't see what the issue is. Often times the savings is a couple hundreds dollars on a long haul. When it's your own money you value it differently than when it's OPM. In addition, the lounges (i.e. the NH lounge at NRT) more than make up for the inconvenience. I don't mind taking a pit stop en-route to SIN from YYZ at NRT to clean up and feel better after sitting in steerage that long. There are also times where splitting it up may make sense from an upgrade sense (i.e. ORD->HNL isn't CPU eligible but ORD -> SFO and SFO -> HNL are). Finally, some of us don't want to fly an AC 777HD across the Pacific and would much prefer flying UA in their free E+ seating.
Safe Travels,
James