Do You Need A Separate Timecode System?
Cutting Straight To It, The Answer Is Yes.
I say yes because of the following when you don't have a separate timecode system like Timecode Buddy, Ambient Lockit or Tentacle Sync.
Most cameras need re-jamming after battery changes - This costs you extra time and if you miss a change then you are out of sync and a client won't be happy.
Timecode systems are getting cheaper and smaller - Gone are the days of big lockit boxes running off 2 AA batteries which would ditur camera people from even putting them on the camera due to extra weight or throwing off the camera balance. Now you can get them as small as a 9v battery with 2 days worth of charge!
You will make more rental money - A separate system means more rental money, OK not a massive amount (around £10-£20 per TC box) but it means you are getting back on your investment and delivering a better solution to your clients.
Conclusion: Everybody Wins*
There are probably more reasons over just jamming timecode with a cable from your mixer/recorder but essentially everyone wins:
You - More rental and better solutions mean better reputation.
Camera - You can set and forget until lunch means less intrusion which is always good.
Client - The most reliable solution with the least stress or expense in post.
Post Production - It will be more reliable so they won't curse your name!
*As long as clients are willing to invest in it when they understand the benefits.