Without secrecy concerning the details during negotiation, it would be very very hard to make trade agreements. Negotiation involves a lot of give and take. If that was all public, then as soon as our negotiators conceded on, say, some term that was going to help our automobile industry in order to gain some term that will help our garment industry, there would be a huge outcry from the automobile industry, and soon the negotiators would come under intense pressure to get that pro-automobile industry term back.

Secrecy, at least concerning the details, during negotiation gives the negotiators a better opportunity to work out a deal that best serves the interests of the country as a whole.

Note that I'm only talking about secrecy during negotiation. The result of the negotiations should be made public well before Congress votes on it, to give the country time to decide on whether to accept or not.