Working with the Sellers Agent only is akin to bringing a knife to a gunfight. You might not get the house you want (because the Sellers Agent want's all the $$$$) but you will have an advocate on your side which is - to be crystal clear - priceless. The builder's agent, although not additionally compensated if you purchase without an outside agent, is working for the builders best interests in every case. Is it possible the builders agent is going to really go the extra mile for you? Depends on the Agent. There are some great ones out there, but when it comes to your needs versus the builder who signs the Agents pay check, who is going to come out ahead?
I'm closing a condo purchase tomorrow (God willing....) that was so fark'd up by the dual agent, the back story could not be believed even if you had the time to hear all of the gory details. Have I seen dual agency deals work out? Sure, rarely, but I can't count the times these deals have crash landed. Having a counselor guide you through the inner workings of a complex financial transaction is well worth it.
Why do the phrases "You get what you pay for" or "A defendant who represents himself has a fool for a client" always ring true? Because more often than not they ARE true. If you have a thick skin that can take bountiful abuse, if you can find peace in the face of likely disappointment, then buying without personal representation might be a way forward.