Yes it does. It reduces the amount of control that the police are able to exert over the messages that the hostage taker receives. You want the hostage taker to be as dependent on the connection to the negotiator as possible. When the hostage taker can twitter, e-mail, look at what's going on in the media, the hostage-taker is both sending and receiving information that is sometimes competitive to what the negotiator is trying to do.
The second thing is that TV coverage always increases the risk that tactical advantage can be compromised. TV coverage may show, for example, the location of some SWAT team members which makes it easier for the hostage taker to find them and shoot them.