They are invaluable for last-minute itinerary changes, at least that's what I've found.

When I used to work in corporate, the company travel agent sorted out serious crap every time it mattered. Once showed up at the airport, Friday evening, everyone on the way home. Surprise snowfall closed the airport and all flights cancelled. Called the booking people and they sorted out a taxi, hotel and other flight in 5 minutes while everyone else was struggling to find a cab. When I got to the hotel, there was a massive queue at reception which I just sailed past. I love a good travel desk.

This also goes for vacations. Why stress over details when you can have someone else take care of stuff?

I know someone with a golf vacation business and a good chunk of their business is via travel agents from Germany. Not sure if it's just bigger over there or if it's more prevalent for group bookings that they cater to for golf.

Your point about keeping corporate bookings honest makes sense. I've found executive assistants to be very good at research and bookings though. But I suppose they could be more easily influenced than an outsourced agent.