External elements should never be made part of the Holy Bible. Far too many laymen are under the impression that external elements are part of the inspired Scriptures. Unless the external elements are markings that lend to better study of the Bible by indicating to the reader the tense of a word or any related grammar of the original languages so as to enable the reader to better understand the author's intent, or marginal notations to inform the reader of variant readings in different manuscripts, alternate translations of particular words, or literal meanings of particular words, there is no call to add subjective external information that colours how one understands the Bible.
For example, many so-called "study" Bibles contain commentary notes that are rife with error and misinformation. Yet, many laymen are under the misguided impression that these notes are part of the inspired text. When they read these notes, they are under the false impression that that is what is taught by a particular section of Scripture. For further example, some English Bibles contain James Ussher's dates. Many laymen are under the impression that such dates are part of the inspired text. Headings in Bibles vary from translation to translation and some are completely false and unrepresentative of the context of the actual text while a few are outright heretical. Laymen are also under the false impression that these headings are part of the inspired text. In fact, most laymen are ignorant of the fact that chapters and verse numbers were never part of the inspired text.
The problem with many modern translations is that they are not honest. Every modern translation should still follow the example of the the Geneva Bible, the King James Bible, and the New American Standard Bible by placing words not found in the original languages in italics or within [square brackets]. By not doing so, these translations are conveying that every word is actually in the original Scriptures. Furthermore, modern translations should also include a notice that informs the reader that chapters and verses were not part of the original Scriptures and inform the reader as to when and why they were added. Sadly, most laymen (and even several preachers) are under the impression that each chapter begins and ends crucial information. Chapters and verses do not indicate the end of one topic and the beginning of another.
If external elements are going to be added to the Bible, it needs to be stated up front at the beginning of the Bible that such elements are not part of the inspired text and that they should be approached with great caution. Cross references are acceptable external elements, but should likewise be stated up front as to their purpose and that they are not part of the inspired text and may be subject to error. If commentary notes are added, especially to Bibles of vanity carrying a particular person's name, it should be stated up front that these notes are not to be understood as inspired, that they are the author's personal interpretation, and that such notes should be approached with great caution, encouraging the reader to study the Bible for themselves like the Bereans rather than simply taking the author's word for the interpretations given. Only the Holy Scriptures are inspired. All else is subject to error, including Bible Dictionaries, Bible Lexicons, etc. The only authority pertaining to matters of faith and life is the Holy Bible. Search the Scriptures and let Scripture interpret Scripture.
Every translation should state up front that chapters, verses, marginal notes, cross references, etc., are not inspired and were not part of the inspired text, and to explain their purpose for the reader. When a Bible is placed in someone's hands, that Bible should be able to educate the layman so that they are not under false impressions with regard to the original text. I long to see an actual study Bible published whose aim is to help the reader actually study the Bible. The Newberry Bible comes close, but it also contains external elements that colour one's interpretation and understanding of Scripture by giving the impression that such things are accurate and true (such as Ussher's dates). The grammatical markings are a great help in studying Scripture, and this is the kind of thing a true study Bible should contain. When the grammar is understood, it eliminates the possibility of false interpretations and leaves the reader with one of two options: obey the truth of the revealed text and conform one's beliefs accordingly, or disobey the truth of the revealed text and reject it in favour of one's personal feelings and opinions. Greek grammar will always decide the case between two opposing beliefs as to who is right and who is wrong, providing both are not wrong.
Why should the Bible contain nothing more than the Bible? Why should the Bible not contain certain external elements? Because it colours one's interpretation thereof and does not allow one to see anything else, and it puts one under the impression that such elements were part of the inspired text.