The MAG is only normally worn during launch, re-entry and for EVAs (Extra Vehicular Activities - space walks). The International Space Station has fully-functioning vacuum-assisted toilets and astronauts are able to keep clean with similar vacuum-assisted showers. MAGs are worn as a precaution during times when access to the toilet is not possible.
Most space walks and the re-entry phase are relatively short periods of time, so although MAGs are worn they are seldom used on those occasions. However, the time spent suiting-up, preparing for launch, lift-off and travelling to the space station is much longer and I understand most astronauts wet themselves during this period. A change and shower when they arrive!