I like threaded conversations.
When I'm in my inbox I can scan down quickly and find what I'm looking for. If emails were unthreaded then one very active conversation could take up the whole of the visible space on my screen.
When I'm in the conversation I can easily glance at who said what and when by looking at the (collapsed) names, times, and first lines.
I don't have to scroll down a load of >>> in the bottom of an email, or go back out to the inbox, to try to look for the last message from Olivia.