It's not completely true that they had "nothing to focus on". Although they didn't have an obvious tumor, they do know the usual "cast of characters" where nodes can be involved and other areas that can cause problems later on. Remember that tumors under 2mm are undetectable by current scanning modalities. It is more of a prophylaxsis type of treatment. Considering the aggressiveness of tongue cancers in gerneral I personally would sleep better at night knowing that they are hitting this as hard as possible. I wouldn't call it a recurrence either, rather a part and parcel of the original cancer (I agree with Mark's opinion).

To answer your original question, they can shape IMRT to any shape they please but the main idea is to avoid unnecessary tissue damage and improve quality of life afterwards. Most of the time Amofostine is unnecessary as IMRT will typically spare the salivary glands unless your treatment protocol puts all of them directly in the radiation field.

I don't believe that neck disections are done for stage one patients as a general rule (surgical resection being preferred) but there are exceptions. See http://www.nccn.org/professionals/physician_gls/PDF/head-and-neck.pdf for more information.