"Musical notation requires more space than a literary text. It is usually read more slowly (the rough approximation of a minute a page for books is, generally, a half to one-fifth of the time required for a page of music), and it is read at the fixed pace specified by the tempo of the music. The process of performance also denies the reader any freedom to re-read, to skim, or to stop momentarily at the end of a line or a page, even for purposes of turning the page. For such reasons, special layout patterns have been found desirable for musical notation." -- Krummel, p. 312
Advantages of oblong format dispersed staff shape note books:
1. there is a longer span of continuous musical text which conveys better the linear construction of the music. (longer staves and fewer line ends)
2. the sight-reader benefits from fewer interruptions in the line of music on a page; i.e. you can read across the page for a longer period of time before the interruption of shifting to the next...
3. the singer can more easily look over the top of an oblong page in order to watch the leader (as opposed to looking over a tall or upright page)
4. the dispersed staffs better display part/voice crossing.
5. an oblong book stays open on its on better than an upright book (e.g. laying open in your lap).
Disadvantages of oblong format dispersed staff shape note books:
1. the oblong page is slightly more awkward to turn (the page-turning hand must travel further; not a problem very often, since there are not that many "page-turners").
2. multiple lines of text are farther away from the line of music.
3. an open oblong book takes up more horizontal space (not usually a problem, unless the location is exceptionally small and the chairs forced as closely as possible).
Some thoughts based on: D. W. Krummel, "Oblong Format in Early Music Books." The Library, Series Five, XXVI, no. 4 (1971): 312–24