The nonviolent activists of today (and yes, they are majority nonviolent, don't let the right wing media bubbles tell you otherwise - I was part of a completely peaceful political protest more than once and absolutely nothing bad happened, luckily for me) such as BLM take a great deal of inspiration from King, and it's an important letter to understand the full context of our times.
In particular, the blasting of white moderate, and of all of those who stay silent and do nothing in the face of evil, is very poignant.
There is no message 'moderate' enough that someone won't holler that it is 'too much': people who kneel at football games peacefully in protest of murder, and get people screaming to have them fired for it are a fine modern example.