The benefit of visually seeing nested replies, is less important than the benefit of having replies in chronological order which then allows to:

• Provide a list of links to the single (only 1 per topic) oldest unread posting in a discussion/topic. No need for multiple links for each unread posting/message, which make that list too long.
• Entering a discussion/topic and reading posting in chronological order.

The only disadvantage of not seeing post/message chronologically (instead of visually nested) is that users replying to another just need to start by a short quote or say who they are replying to if it isn’t obvious (like the message just above). This doesn’t seem to be a problem on the international Paragliding Forum, which provides the ideal list of links to the oldest unread message under each topic (click this  to see). The attached image  shows a typical result, and you just click the orange “UNREAD” link to get into that topic’s oldest unread message, great!