I'm not a big fan of line branches after having lived on one: the "M" Myrtle Ave. line of the JMZ in Brooklyn. On weekends, as a cost savings measure, they would take advantage of the branch to run a shuttle service to the branch point, where you would have to get off and wait for the next "J" train to come to take you into Manhattan. On weekdays, a train on a branch would have to wait until the other track was clear in both directions before proceeding to merge, holding all other trains behind it up as well.
Of all the subway systems, New York is probably the most branched and interlined and it can be extremely nerve-wracking and disorienting even for seasoned travelers to have a train reroute onto another line on weekends or late-nights. Right now, there's this Bermuda triangle somewhere in downtown Brooklyn where, mysteriously, the F train becomes the G, the A the F and the J backtracks as a shuttle on the R line. Moral of the story? No branched lines.