"Neocon" leanings and all, I just need to say it: the harpsichord is a sucky instrument. I mean it's ok as part of a band I guess...there's something to that velvet on top/spiky on the bottom texture which one comes to expect, and the times I've heard, for example, a Brandenberg Concerto with piano instead of harpsichord, while cool, it is a big difference.
But as a general rule, and especially taken alone, it sucks. I don't know anymore than the basics, but there's definitely some evidence that Bach was into the pianofortes which all the early-adopting Princes were picking up (the improvisation which birthed the Musical Offering was performed on a pianoforte, legend goes). Knowing this, and also knowing Bach was obviously a man of taste, do we really have any reason to assume he would have been all "Oh sweet, I'll take this keyboard instrument that sounds like steel wool with pitches, you just keep that thing that allows for like the most exquisite fine grained expression any intsrument has ever offered." Yeah...o-tay.
Thus, it is kind of a bummer that the BBC programmers seem to have caved to the period instrument mafia and are emphasizing harpsichord recordings over the catalogue of Bach on piano (Play da harpsichord or we break you. Do it.) Thankfully, I am currently in an Angela Hewitt patch...but the harpsichord, and the brain hurting, is bound to return.