Travel can be a net productivity gain or loss. It depends on what travel is used for. It also depends on duration.
For actual work, travel produces a net loss. The hours lost due to travel, the time zone changes, and the logistical friction of working in a new city produces immense friction for actual work. The net loss changes the longer one stays in a city, but for short durations the above is the case.
For networking, travel is a net gain. Despite skype, email, and other forms of interaction with people, nothing replaces face to face interaction to bridge trust. Also networking is an activity that requires substantially less cognitive load. Lastly, meeting with other people that traveled to the same place is a great way to bond over being stuck in a place together.