There are occasional scholarly disagreements over the performance practice of Bach motets-- whether they were meant to be performed with accompaniment or without.
Why is this important? In this day of stringent scholarship, performers can come under harsh criticism if their rendition of a particular work does not adhere to the "performance practice" of the period. That isn't a bad thing, to be sure, but it can occasionally inhibit conductors-performers from attempting music of the early periods.
In doing a little research this morning, I found a couple of places that argued for their performance to be without accompaniment.
From a 1998 discussion on ChoralTalk:
Daniel R. Melamed, "J.S. Bach and the German Motet." This book contains the most recent research about the Bach motets . . . .
. . . .It is also now clear that although instruments were banned during the funeral rite at the Thomaskirche in Leipzig, "Der Geist hilft" (for which not only continuo parts but also parts for instruments to double the voices were supplied by Bach) was performed at a memorial service at the university (where instruments were not banned). The supposition is that the Thomaskirche rule on instruments for funeral rites was the only impediment to Bach's composing instrumental parts for all his motets. The conclusion surely is that the motets (except for "Der Geist hilft", "Ich lasse dich nicht", "O Jesu Christ, mein's Lebens Licht", and various cantata movements in motet style) can be performed either a cappella or with doubling instruments. They work either way; one should not attempt to maintain a purist attitude in the face of the existing evidence.
I also found this discussion on www.bach-cantatas.com. I've blogged about it before. This particular discussion concerns a work we are not performing but is in the same body of works by J.S. Bach. General details about the performance practice of Bach motets are a little further down in the quote:
Indeed, the parts for "Jesu meine Freude" are not by Bach. Moreover, recent research has determined:
1) the singing of Bach's motets a cappella is not a romantic-style innovation of a later period than Bach, but rather can be documented as the manner of performance of some of these motets (a carrying on of the tradition that was already established under Bach?) under the cantorship of Johann Friedrich Doles who conducted/performed these motets in this fashion at St. Thomas Church from 1756 to 1789.
2) new evidence that some of Bach's motets, such as this one, were very likely performed a cappella without colla parte instrumental accompaniment or bc
In Klaus Hofmann's new book, "Johann Sebastian Bach: Die Motetten" [Bärenreiter, 2003] pp. 55 ff., the following evidence is presented:
1. in 1761 and in 1764, the music catalogs of printed works available from the publisher Breitkopf in Leipzig offered copies of Bach's motets in the category "Motets - without instruments"
2. the music theoretician, Johann Adoph Scheibe (1708-1776) (his father, Johann Scheibe, an organ builder was very highly regarded by Bach) is the infamous critic of Bach's music in the Birnbaum-Scheibe controversy.
To consider his attitude toward Bach to be entirely critical would be unfair since he was also one of the very first to praise in print Bach's 'Italian Concerto.' Scheibe was encouraged by Telemann to publish a periodical, "Der Critische Musicus" ("The critical Musician"). From a reprint of an issue of this periodical dating from 1737 [Hamburg - he had just recently moved from Leipzig (where he had had most of his schooling including some time at the University of Leipzig, until his father's organ building firm went bankrupt)] but reprinted in Leipzig in 1745(facsimile Hildesheim, Wiesbaden, 1970) pp. 181 ff., Scheibe comments on the actual motet performance practices he had experienced in Leipzig:
>>Der Generalbaß sollte zwar allezeit dabey sein; allein, man kann ihn selten gebrauchen, weil die meisten Motetten nur von einem Chore Sänger aufgeführet werden, es müßten denn andere Instrumente mehr dabey seyn, oder man müßte sie bey gewissen Gelegenheiten in der Kirche aufführen.<<
Klaus Hofmann interprets this passage to read: "In reality, a basso continuo ought to be part of this ensemble, but the general performance practice/tradition rarely makes use of this option, because most motets are performed only with voices. It is, however, an entirely different matter if more instruments (beyond the colla parte and bc accompaniment) are used, or when the motets are performed in the church on very special occasions." Klaus Hofmann also extrapolates from this passage the fact that a cappella performances must have been common practice under Bach's tenure in the 1730s.
3. there is no record of any original instrumental parts for "Jesu meine Freude" as ever having existed at any point during Bach's lifetime.