At some point in time, you will be given a task like “What happened to X? Why can I no longer see it?” or “Why does the date on X say ‘Dec. 31’ when it previously said ‘Jan. 1’?” With any sort of complex system, these kinds of questions are inevitable. A user changes something without telling another user, or a weird process messes with data, or worse yet, a bug happens (which is almost inevitable).
With if you are properly logging an audit trail, these kinds of questions become really easy to solve. Want to know who deleted X? Check the logs. The logs don’t even need to contain all the detail – just a general idea. For example “User ‘Gordon Freeman’ deleted X on Aug. 1, 2012” is detailed enough to tell you who did it, and when it happened. It’s so much easier to add some basic logging in at the start, then to spend time trying to track theses kind of things down after the fact.
Save yourself some time. Add logging/auditing right now.