Key problem of the new @reply redesign on @twitter is: you can't avoid replying to the original twitter user.
it seems #twitter implies that a contextual reply is always done towards the the original source of the tweet.
in mobile and desktop you can untick the users you don't want to include in your reply but you can't untick the source of the tweet from the conversation.
Why would anyone do that ?
There is plenty of very valid reasons to do so :
use case :
Imagine you find a tweet you want to relay to a bunch of people, but you don't want to necessarily reply to the source of the tweet itself, before the change you would simply remove all the users from the reply and add new mentions to the users you want to include or start a new conversation with. now you can remove all the users from the context less the source of the tweet.
Imagine you are being harassed by some trolls and you want to reply but without including the person being harassed (she's already on too much) or you want to help her or him and divert some of the trolls back to you, how do manage that with the new reply system ? you can't. in this case #twitter removed options for victims of harassement on twitter.
I'm still analysing what the new reply system will change, mainly regarding context analysis for #dataviz and other advanced use of Twitter.