There are a couple of reasons I can think of for keeping your military time in your civilian log book.
1. Civilians log time block out to block in. Military logs t/o to land. So the typical civilian logbook contains more time for the sam flight. If flying as a civilian pilot is in your plans after the military, this extra (and legal) time might come in handy. Some airlines like Southwest have former military guys add 0.3 hours to each leg in their military log to as a conversion factor. Most don't. Further most corporate flight department do not either. Keeping your military time in a civilian logbook just might get you over a minimum hours requirement for hiring or insurance.
2. At many military airfields, it takes a lot more than 0.3 to taxi out for t/o, get an IFR release and taxi back after landing. Southwest's 0.3 addition is the typical conversation factor for those that allow it. You might be short changing yourself towards an equivalent civilian time.
3. It's nice to have all your records in one book in chronological order. This way prespective employers do not have to jump between records when reviewing your logs. Keep and take your military logs as "proof" and be ready to explain the difference in logging leg times (out/in versus off/on from #1 above). Since you kept a civilian log, the prespective employers will accept the longer leg times as they will assume you keep track of actual times and did not just add a conversation factor.
4. If you lose your Navy logbook, you are not at the mercy of Navy record keeping to try and reconstruct it. I know of 5 or 6 guys this happened to.
Mefesto said:
Exactly... down the road that may bite you in the ass... logging the time twice.
You're not logging the time twice, you're just keeping two records and differentiating between military and civilian logging methods.