If the home is still under construction there are several layers of paperwork including the original contract,sometimes two closings (a construction loan and final mortgage), lot selection and choices of options within the home which have to be well documented. There are opportunities for things to go wrong. And in Maryland the agent that is hired by the seller to sit on their property and greet people most definitely represents the seller. They are obligated to treat all parties fairly but they are not your advocate. When making a decision and following through on what will likely be your largest purchase ever you'll really want someone there that is just in it for you!
I disagree with the 1st agent who answered this question. Perhaps she is speaking in terms to Florida real estate practices; when she says the listing agent is representing the transaction; not the seller.
Since this question was asked under "Maryland"; I can tell you that in Maryland the listing agent (the "on site" agent) IS in fact representing the seller, and working in the sellers best interest. What you should have is a "buyers agent"; who would be representing YOU, the buyer.
Now there is some tricky stuff if another agent showed you the property first. The 1st agent needs to know that because there could in fact be a commission dispute. However most listing agents (or at least their broker) will back down if faced with the possibility of losing a sale if they dont allow you (and compensate) your buyers agent. Afterall, they must work in the sellers best interest and losing a sale because the listing agents wants both-sides of the commission, is not in the sellers best interest.
good luck with your search
Many realtors will tell you that it's fine to use the listing agent because they are representing "the transaction" not the seller. My personal belief is that there is a conflict of interest when trying to negotiate the best price for the buyer when the seller has listed with you. Agressive negotiations have become especially relevant in this market. For this reason I would always have my own representation, not the seller's listing agent.
No, you should not bring in another agent once you have had the on-site agent show you property. If you merely picked up some info, it's fine but if the on-site agent has shown you property then you need to disclose this to another agent that you might hire because they could find themselves in a commission dispute.
My advice is to find a good realtor that knows your market and will be a "bulldog" in negotiating the best price possible. Tell them the areas that you are interested in and let them represent your interests only. If they only offer to show you properties that are listed with their office, then move on. There are also "buyer's agents" that have no listings. Good Luck