True, on a spaceship (as it is with any sufficiently complicated machine) there are multiple vital systems and persons. One thing falls and you're in serious trouble. This means, that with a very large ship, with thousands of people, there are too many high priority targets for the security to reliably look after. Following: if players know, that something nasty is going to happen (for example catch a loose note about a planned sabotage), they won't be able to just fortify in few key structural points (with all important persons) and run a tedious and time-consuming security scan of the whole crew. They will have to actively hunt down potential criminals.