You should definitely use him/her 1) to reward them for their service and friendship 2) to get some money back from their share of the commission (from the builder), which is otherwise NOT paid to you.

But, be careful–you must sign the contract with your agent present, or have made the first visit to the model homes with your agent present. The agent cannot come into the picture after you expressed interest to the builder. The builder only pays the realtor comissions if THEY played a role in bringing you there.