IMHO articulation is a non issue for Bach. What makes me shudder is improvised ornamentations, as in the kinds that conjure notes out of nowhere.
Whether or not these rules have solid historical background.....This is my understanding of current Bach performance practice...in terms of articulation. many of these rules stem from early keyboard technique, as applied to harpsichord, clavichord etc, which do not have the sustaining sound or sonority of the modern piano. Rhythmic and tonal clarity take precedence as overlying interpretive effects. 1) Longer note values more detatched than short values2) Leapwise motion more detatched than stepwise motion3) major cadences always detatched in bass outline (5-1) (5-6) (2-5-1) ect. -Especially important to break up any percieved monotony (for the listener's sake)4) motivic units played with consistent articulation5) Detatch before syncopation6) Detatch upbeats7) Detatch before subject entries. pedal used judiciously in counterpoint.This is only a partial list, and of course, only a fool would allow these to become a straightjacket. These are the expectations, and artistry is additionaly shown in how the performer approaches these rules. Any of these "rules" may be broken for a specific musical reason. Hovever, if the student has no convincing reason to break these rules, it is advisable to use them as guidelines.