The use of managed modes to improve situational awareness can be a double-edged sword. Many people misunderstand is as "go managed at all times to improve your situational awareness". And it's not that clear-cut. The use of managed guidance can pay off in a situation where you've had the chance to instert everything in advance and then just monitor its execution while focusing on the bigger picture. But fiddling with the MCDU in descent instead of flying the aircraft is neither good airmanship, nor a precursor to good situational awareness. If it doesn't do what you want it to do or the situation has changed (e.g. change of routing invalidating your lateral flight plan) - go selected, focus on flying and, if necessary, ask the PM to sequence the flight plan whenever his workload allows for it.