I don't think people are resigning to send a message, they are resigning, and also sending a message. If you're working for poor pay for something you believe in, and all of a sudden management is making it impossible for you to do your job, burn out is inevitable, along with leaving for a higher-paying job in the private sector.

It took out Nixon: https://en.wikipedia.org/wiki/Saturday_Night_Massacre

Unfortunately, most of those committees don't have any actual power to enact anything. By resigning in protest, they at least prevent themselves from being used as justification to enact the things that the Trump admin is doing.