We jump all over our peers when they blatantly self promote while adding little or no value in their answer to the question.
What we saw here, amongst five posters before you, were good answers, including the horn toot. The poster who received best answer by the questioner held her horn tooting off until after 4 paragraphs of great advice. - Even then it was subtle enough for me. - And I am a rather harsh critic of some of the self promoters.
Have you ever heard horror stories from someone you know who purchased a new home? No one interpreted the purchase agreement for them, they were not informed of certain upgrades available to them, no one was their advocate with regard to making repairs before and after the sale...the list goes on.
Now if you have identified a development already, there is a chance that they will try to deny you your own representation. Most builders in this area require that your agent be present with you during your first visit to the property, and fill out some "registration" paperwork in order for your agent to be compensated by the builder. If you do feel an agent would assist you in your purchase, you may not want to waste time finding one you like.
The listing agent is put in a difficult position by representing the buyer and the seller. There is no way they can truly represent the best interest of both parties with conflicting interests.
Congratulations on finding your property. New developments are usually marketed through their sales staff (as you mentioned). When they are closing out their development and have a just a few left and/or when they haven't been successful marketing it themselves is when they would generally find a realtor to "list" the property. I'm just wondering what you mean by "marketing agent", I'm assuming that they have a listing agent.
The advantages of using a buyer's agent are many especially in most cases the services are provided to you, the buyer for free. Finding the property is generally the easy part of the buying process, the rest that follow are complex and time consuming. You are hiring a professional to represent you and your best interest. I would find a realtor to further discuss these properties with them. Won't cost you a dime to explore the possibility! Good luck!