Is there any reason for them to appear?

Didn’t the GQP establish that Congress has no right to subpoena anyone?

Did Meadows appear?

Isn’t the course of action to appeal then appeal the appeal, until Congress changes hands?

I don’t see much upside to appearing. Since there is no evidence, the hearings will be like the Benghazi Hillary hearings, where she never was asked a question, but just sat there while Republican after Republican yelled sound bites for Pravda.