1) The deposit. Good luck getting it back from the builder if the deal falls through! You may have to pay a lawyer to sue them, and spend more than the amount of the deposit trying to get it back. A Realtor can have the money escrowed at the brokerage, which is a lot safer for you -- nobody gets it until closing or a release is signed by both parties.
2) Builders may give you trouble with the inspection, or even claim you don't need one. You do! I've had new construction clients whose inspections found egregious faults (e.g., several pipes jutting 2-3 feet out of the middle of the floor in the "finished" basement). A Realtor will take you through the inspection and walk-through process.
If the builder won't pay the commission, you may end up paying it (it would only be "half", and might be well worth it if you really want the house and the Realtor helps you negotiate a good price). If you're determined not to use the Realtor, you can always just wait out the time period of the agreement. By default, the agreements last 1 year, but usually agents write in shorter periods (e.g., 90 days).