Interesting but I see it as a positive move to improve DOT on-time numbers. Sure beats cancelling one of the legs and flying on to next destination.
As the flight showed on-time any LAX-SLC throughs were probably rebooked prior to departure at origin. Same with LAX outbound connections.
How they managed to get crew and aircraft in LAX I do not know as I don't see any cancels but then again it was Saturday so there may have been a spare aircraft at LAX with the usual reduced Saturday schedule.
Per flightstats.com it appears the LAX-SLC leg of WN3125 actually flew using a WN9007 call sign to prevent duplication.