Hello again, Robert -
I believe that you hit on one point that I was more or less dancing around:
We who hold this position do not believe we must follow the Jewish and Roman lifestyles of the first century. Just because Paul might have worn a toga & sandals and walked to church does not establish that practice for us.
One could argue (though I have no desire to argue with you) that the elder system we see demonstrated in the New Testament was an adaption of the Judaistic system of "church" polity and necessary for the times and past situation in the emerging church. As times changed, so too did the polity of the church. The council of elders in the Gospels and the Epistles is described as a group of men who acted for the people as a whole. In fact the Jewish elders were quite instrumental in bringing about the crucifiction.
It is not until Acts 14:23 that we see Paul and Barnabas appointing elders. All mention prior to this indicates an already established group. Further support that these groups were not clerical can be found in Acts 23:14 in that they are listed as a seperate office from the priests. 1Timothy 5:17 shows us that a person can hold two distinct offices, that of "preacher" and "elder." Aside from your citing of Acts 20:28, the only other "job description" I can find for an elder is in James 5:14, that they pray over and annoint the sick with oil.
I will go a little further out on the limb here. I believe that within the church, just as within any social structure, leaders will naturally emerge. Even in churches that do not exercise a system of a plurality of elders have that core collection of individuals who rise and see to the proper workings of the church. This is just my own personal observation, but just as David was commissioned by God long before Samuel's annointing, so too does God place people where they need to be in His church to perform the necessary tasks.
Paul and Barnabas were left with a task of appointing elders because the infantile church was prone to straying from true doctrine. Once the Apostolic age passed (I know that's a debatable point) and Christian doctrine and polity was established, the appointing of elders was not a necessity. "Elders" come to the front now when the need arises to correct doctrine but otherwise, the office is a redundancy within the church.
As to the laying of hands, I would be very interested in exploring the divergence you spoke of between the "5 principle" and "6 principle" Baptists. The laying of hands, like the council of elders, is easily traced to the Old Testament as an inherited Judaistic practice. The earliest mention of such a practice is Genesis 48:13-20. Just as Christ used the practice for some of His healings to show from whom the power imparted, so to did the early (and modern) church use the laying of hands to represent authority and succession. I don't think many modern believers look at the laying of hands as deeply as they once did. I am SBC and I have only seen the practice used for ordaining deacons, pastors, and the occassional infant dedication.
Finally, I would like to clarify a position you took on Sunday School:
In my church, Sunday School is not just for children. The class I attend has two octogenarians as members, in fact, Margie and I are the youngest in the class. In that setting, we explore Scripture much in the same way that we are doing here, with close careful scrutiny. Jen (my 10-year old step-daughter) would have glazed eyes by the time we got to the second sentence and would not get anything out of the discourse. For an example, if one week we were exploring the subject of David's life, it would be appropriate for the adults to explore the nature of David's affair with Bathsheba but it would not do for the kids. They would learn about Goliath or the return of the Ark. I personally feel that this is appropriate. Sunday School allows for interaction and participation from each member. It is a study of Scriptures by the letter. The message preached in the worship service, on the other hand is a single narrative based on the Scriptures and expounded to an application to the present believer's life. The format does not allow for the same Q & A period that Sunday School does. I think that Sunday School is easily justified by such passages as Acts 17:10-11. Studying the Scriptures in a group to look for evidence and truth was cited as a "noble" practice and thus the Bereans are held as examples to us from the New Testament writings.
I fear that my posts are not addressing the issue of "why" we diverge as much as they are merely defending the practices I know as "normative" for me. Maybe once you set the hook I can remain more on target.
To expound further, another historic variance in Baptist thought is certainly the missionary split of the 1830's over evangelism. Still another that was born of this divergence is the use of conventions. The justification for that is, of course, that it is an effective way to implement the Great Commission. However, if one interprets the Great Commission as an Apostolic charge from Christ rather than a charge to the entire body of believers, or that the Commission is given as a direct command to individual believers rather than a delegated responsibility, the point is moot.