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.
From leaked documents, it appears that inside the negotiations, there's really not the kind of secrecy you're imagining. Every part to ACTA, for example, just marked it up as they thought it should be, and then there was a lot of something (not public) to get to a final draft. That's not keeping a maximum (or minimum) position secret in order to gain advantage, that's just deciding how much of something can be foisted upon the global public without causing a revolution.
From my standpoint, the secrecy we get is of no use to anyone but corporate fat cats and insiders.