Can someone explain the purpose of decomposing to me? I don't do much conforming so it's not something that comes up often for me, but I can't see the point of it. If you select a sequence and click batch capture with no handles surely you get exactly the same result, no?

Excuse my ignorance but I don't do much onlining. While I always thought the correct way to do it was to decompose, the online editor who's just asked me to do a quick conform or them said 'it's up to you, it makes no difference' when I asked whether I should.

Well it makes no great difference to the end result but what it does allow you to do is check all your reels / sources prior to beginning what may be a lengthy auto-conform.

For example if the offline was a long drawn out process in several locations/suites with mutiple tapes/sources and perhaps several editors, the last thing you want is to sit into an online session and not have a complete source inventory and all the relevant sources immediately to hand. Be they on tape, hard drive or from graphic sources. Simply choosing to Batch Captue won't allow for that.

Whether you choose to add handles or not has no relevance either. By adding handles you give yourself the ability to add mixes , wipes, etc which may not be in the original cut. Without handles you can't do that.

With bigger harddrives decomposing is not needed as much today.  When I used to work on longform projects on Avid Meridien systems, I would digitize hour long and 90 minute tapes at a lower resolution (15:1s).  This would take up very little space.  Once we finished editing our hour long show, I would duplicate the sequence and decompose only the video in the timeline.  I would then batch digitize and batch import to a higher resolution (2:1), and replace the low resolution video with hi res video and then render.  I would normally select 30-60 frame handles in case I had to do some last minute trims.  

It's true that decomposing now is much less needed. However, when Avid first was able to deliver resolutions that broadcasters would accept for on air use in the mid 90's disk space was still bulky and quite expensive, with the average Avid system having a total of 36 GBs of media storage. I was editing show episodes for a series "Country Inn Cooking" at the time and I'd have to have as many as 5 different episodes in varying stages of completion on the system at one time. I would work extremely low rez (15:1) for the first cuts, with only room for one show at high rez (AVR75). Decomposing proved invaluable for dumping low rez media for the final approved cut and batch capturing at high rez with minimal disk space used.