MBTA officials say rampant absenteeism is hurting bus and subway service, discouraging riders, and eating away at the agency's finances by forcing it to pay overtime. ...about 35 percent of the Massachusetts Bay Transportation Authority's roughly 6,000 employees were absent for 11 or more days in 2004, and 16 percent were absent 26 or more days. Khalida Smalls, coordinator for the watchdog group called the T Riders Union, said yesterday that excessive absenteeism directly affects passengers. ''People stand at their bus stop at 4:45 waiting for a bus that doesn't show up," ...