Well, there are many arguments either way, but I think the real question is can we ever re-create Bach's environment, and, with that, Bach's listener?

It would be BEAUTIFUL if we could find boy sopranos who are schooled in Bach's music to the point that his own choristers were, but that is simply not possible. Those young men lived with Bach's tuttledge every day of of their lives -- there is no way to recreate that.

In addition, we have no idea what sort of tone was created by those young men. I think it is particularly harmful to think that they created the typical "English" chorister tone -- this is a non-continental creation which has no relation to Leipzig in the 18th century. Listen to contemporary German Knabenchors (Windsbacher, for one). VERY different tone.

As an example of using women to GREAT effect in Bach, try this YouTube link. QUITE magnificent:
http://www.youtube.com/watch?v=7lWXRUwdXrk