I've been astonished over the last few weeks at how many Fundamentalists think it's a good idea to have Calvinists (including 4-pointers) and Non-Calvinists (however that may be presented) in the same groups. If we for a moment leave out the mid 19th to 20th century, this seems extremely odd to me. The early Protestant Reformers did not allow latitude on this issue (Luther and Zwingli held doctrines that would now be considered broadly Calvinists). When the Lutheran Church under Melanchthon deviated toward a mediate position, the Reformed congregations considered them "other". When the Remonstrants sought to modify Reformed theology, representatives from all the major Reformed nations condemned them at the Synod of Dordt. Throughout Baptist history, there were both General and Particular Baptists - but they were separate from each other. The Methodist Church quickly separated into the Calvinist and Wesleyan Methodist Churches.
The admixture seems to have begun among the New School Presbyterians, some of whom took some eccentricities in Edwards' doctrine and magnified them into a Pelagianizing doctrine. Charles Finney wedded this "New Haven" theology with Methodist practices to produce revivalism in the mid-19th century. It is clear, though, that the results of that merger extended beyond what anyone expected.
How one answers the basic questions of depravity, regeneration, and election has innumerable consequences - potentially different views of justification, sanctification, assurance of salvation, proper evangelistic methodologies, proper preaching methodologies, proper apologetic methodologies, etc. It is obvious that the "Calvinist" controversy is much more significant and far-reaching than whether I wear jeans or khakis to church, or listen to Fanny Crosby instead of Kristyn Getty, or whether I attend prayer meetings with the local Southern Baptist minister.
Since we have the chance, why not separate over something important?