That always bugged me. The transporter's sole purpose is to move things from point A to point B as efficiently as possible. So why not just beam places? Maybe protocol is to discuss the mission while en route to a transporter room. The actual answer--and yes, there is an actual answer, as hardcore Trek fans will have undoubtedly been muttering until now--is that site to site transporting is essentially a pair of transports back to back. Thus, if policy followed convenience, there would be two essentially independent transports taking place for each single instance of transportation. You thought there were a lot of contrived transporter plots before, how many will there be if there are twice as many transports?