In rape cases there can be a bit of she said he said and such testimony can be important. However, it's up to the jury to decide how much weight they attach to such a letter.
If one of my close friends was accused of rape and I never got any signals that she/he was capable of doing such a thing, I think I should be allowed to be heard. And if I was falsely accused I expect those that know my character to offer the same.
If I ever get the request from someone I have any doubt about they're getting the polite middle finger. But if I'm 99% sure they wouldn't do such a thing I guess I at least owe them to share my pov with the jury. Up to them to weigh everything.
In this case the actor who portrayed Mr. Red has shown he's a bad judge of character but it shouldn't stop us from using the character of the show to make cheff's kiss memes (not this one though)