I wasn't the person that made the claim - but since I feel the same way (well, maybe not quite as strongly!)... I find I often lose track of earlier messages in a thread that I haven't yet responded to, because I reply to some other message from someone else who replied later. I also find that other people make the same mistake and as a result fail to reply to my emails to them sometimes.

I find emails much easier to deal with when the actual emails are in the list, rather than the conversations - although when displaying the message there's no reason not to show it in the context of the conversation.