Earlier quoted context omitted.
The problem with letting users choose the view is that the chosen view affects how they write. If I can see it's obvious in my tree view who I'm responding to, I won't point it out, and then someone who uses the firehose view won't be able to follow the conversation.
I feel the main problem wouldn't be this (if you handle quotes to show in a tree view then surely you can handle replies in a tree view to show quotes in a firehose). However, the main issue is that when you can quote multiple comments (when you are in a firehose view) you tend to send a single post for multiple replies; I have no idea how that could be shown in the tree view. Essentially a firehose is a superset of…
The potential for problems would arise when everyone Replied using a root-level post to the firehose. This could be mitigated with prominent Reply links on every post in the firehose, with the root-level contribution form deemphasized. So you can quote multiple people but you still chose a parent by choosing which Reply link to click.