What are the benifits of a design and build firm with an architectural design element in house? 

These companies still have to hire an architect or an engineer to stamp drawings. 

You said it.. Design AND Build. One stop shop for designing through actual construction. However this comes with it's own challenges. It's more of a developer role verse just being an architectural firm which provides professional design services. Huge difference in insurances being a General Contractor as well as a Design Professional. @Nathan Krause    

The Design/Build model is a unique niche that a lot of people claim they are but don't really meet the definition. A true D/B team means they have an architect in house for the design part and a contractor and their team in house for the build part. I've seen a lot of companies that claim they are this but then contract EVERYTHING out to other firms/businesses. Anyways, back to your questions. 

The main benefit is that you have constant communication from the start between the design team (DT) and the construction team (CT). This allows for both sides to know what each other are doing and thinking which allows for a lot of the problem solving to be done at the beginning of the project (during design) and not at the end of the project (during construction or even after that). Most contractors, in a traditional contracted role as the builder, are not as willing to do this and if they are its usually minimal effort. They make a LOT of money off of design mistakes, design changes and any other types of change orders. The more honest ones will try to keep this to a minimal but even they cant eliminate all of it, no ones perfect. BUT if they are in-house and a part of the team from the beginning they help bring those changes and mistakes down to a bare minimum. Plus when their pay is based off a salary, and not the traditional profit making methods, their only motivation is to bring costs down as a whole and not worried about making money on change orders when they do occur.  

Another feature is that, because of this close relationship from the beginning, you are able to project costs for a project much better and more accurate. From a developers perspective this isn't as big of a pro since you are basically your own client but it is still a very god feature. Cost projections are always important. 

MOST places in the US, and I stress MOST, only require either an architects or a structural engineers seal on the drawings. Most experienced architects will feel comfortable do structural calcs for a residential house. Its usually not till you get to very large, mansion style homes, with crazy designs that call for lots of steel that an architect will defer to a structural engineer for design. But the typical suburbia home thats ~2500 sf ish is not going to be a problem for an architect and MOST cities will except the architects calcs and seal. I will add though that the large developer I worked for contracted out ALL their structural design. They had the volume and house sizes to have a standing contract for this and it made it cheaper to hire that out than have an in-house person. Plus they also did commercial projects, like 5 story,  200 unit apt buildings that no architect would ever touch to do struct. calcs on even if the city would let them (which they wouldnt have). So at some point the economy of scale and size does kick in and make a structural engineer worth it.

NIk is spot on but a good GC can operate as smoothly as a builder that has an architect in house. Just make sure if you're looking to operate this way, or you're looking to hire someone that claims they are both; look into their systems. Specifically, I am an incredibly systems oriented guy, I have systems for every tiny aspect on a build and we track our costs down to the nail, with scheduling and sub-contractor management; we operate the same way.