[She feels bad for Ezra, she does. It's not so much that she feels he's useless in a fight against Vader as it is that she doesn't want him to fight against him. Granted she doesn't want Luke to fight against him either, but she knows full well that Luke will do what he wants and go after Vader anyway.]

I've heard enough stories about him. [she's trying to be patient and understanding, but there's a obvious note of restraint to her tone. Because listening to anything remotely positive about Anakin Skywalker is not something Leia likes to do. But Ezra is in a delicate state, and he's still her friend (she hopes), and she doesn't want to upset him further.]

I don't know if they'll be able to beat him in a fight, but I know if there's even the slightest chance of getting through to him [not that Leia thinks there is, but that's not the time for that] then Luke, at least, will be able to do something.