Would be extremely valuable for people who come under attack, I imagine. A lot of people who are obscenity averse might like to enable it too. Heck, I might even enable it myself when reading the activity on some hashtags.
This is a social problem without a technical solution, even not permitting blocked people of following and retweet like she'd like to is of litle use when it takes seconds to create a new account...
On the bright side, I don't think there is much in the way of following up on those threats in meatspace, cowards will be cowards...
The issue here is that the offensive text is so close to the desired text, to the point that quoting threatening material makes it non threatening. The tweet 'He said "You will be raped tonight"' may be as non threatening as it gets, while "You will be raped tonight" should land its author before a judge.
I'm not saying it's impossible, but it's IMHO very hard.
But a few things make it easier too:
1. Twitter can see all messages from the sender -- if the stream contains a lot of suspect material and was created recently (for example), that makes any individual Tweet more suspect. No individual mail client can do this. 2. A 'false positive' in Twitter has a lower cost than in email. People ignore tweets all the time, so hiding the wrong thing sometimes won't matter so much. 3. Twitter is a more constrained platform, so the problem domain is presumably more limited.
I don't mean to say it's easy or even "not very hard" to pull this off -- just that it is either well within Twitter's capabilities, or soon could be.
The real issue here though is that these trolls know that there will be no legal consequences for their actions. That needs to change for anything to change and Twitter does have a key role to play in that... they're just understandably reluctant to do so.
Given that there are real costs attached to addressing the issue (not just legal but also in terms of engagement statistics) as a rational business they will probably delay addressing the issue until the costs of ignoring it (in terms of bad publicity) exceed the benefits.