Yes, I fully agree. If they are not in communion, then whatever they disagree on can't be likened to eating crackers in bed. :) And of course you are right that there are certain core beliefs among Protestants that link us together if only to define what Christianity is. Unfortunately, many claim to be Protestant Christians when their beliefs do not meet even the most basic Biblical standards.
Whether they admit it or not, many so called "Protestants" today are functional Roman Catholics and semi-pelagians.
RC Sproul has a marvelous book on this topic titled: "The Pelagian Captivity of the Church".
We can trace the slow infiltration of semi-pelagianism from John Cassian onward to today.
Very unfortunate, but very true. Thanks much for the book recommendation. :)