Yes, the time commitment of having one person pickup the phone and place 100+ phone calls (and the suspicion on the other end when you call back with a new name but the same voice).

You could write a screen scraper to book online through the various booking systems, but each booking system probably has its own restrictions on how many accounts you can have and how often they can book. You skip all of these protections when you phone your reservation in (arguably, the restaurant staff should be enforcing these protections when they pick up the phone, but restaurant staff are often overworked and apathetic).