As per the Article 8, comma 4, of the "EU Council Decision of 23 September 2013 on the security rules for protecting EU classified information":
- Areas in which EUCI classified CONFIDENTIEL UE/EU
CONFIDENTIAL or above is stored shall be established as
Secured Areas in accordance with Annex II and approved by
the competent security authority
It is safe to assume that the "secure reading room" in which politicans can read the text is a Secured Area, and thus that the TTIP is classified as CONFIDENTIEL UE/EU CONFIDENTIAL or above. This means that the unauthorized disclosure of the TTIP would arguably harm the essential interests of the European Union , rather than simply being disadvantageous to the interests of the European Union. Note that there is an Agreement between the USA and the EU on the management of classified information (see http://www.state.gov/documents/organization/105689.pdf) in which each Party recognizes the classification levels of the other, which means that - at least from a legal standpoint - my argument makes sense.
Clearly, we are talking about economical, financial, and/or industrial interests of the European Union, but we can only speculate about the reasons why the disclosure of the TTIP would harm these interests.
The United States are currently discussing another, apparently similar, treaty: the Trans-Pacific Partnership (TPP). According to the US Trading Representative Ron Kirk:
[T]here's a practical reason [to not disclose the TPP], for our ability both to preserve negotiating strength and to encourage our partners to be willing to put issues on the table they may not otherwise[...]
By the way, Wikileaks has disclosed part of the TPP (specifically, the intellectual property chapter), and it is marked as "TPP CONFIDENTIAL", so it is possible that the TTIP is marked "TTIP CONFIDENTIAL". This would make the situation a bit different, since "TTIP CONFIDENTIAL" could refer to information that might harm the interests of the treaty itself, rather than those of the Parties.
As a result, I think that the TTIP might include measures which would not be welcomed by a part of the EU citizens and/or by some organizations, but would probably benefit the US and the EU as a whole, and that the resulting protests might put at risk the signing of the Partnership.