3 wheeled vehicles have been around for a long long time and will continue to be for the simple fact they cut size, weight, and costs. For a small commuter vehicle that does not need to drive over 100mph they can be a very attractive option. They can be designed in many different ways which is why they often get misjudged as dangerous or easy to flip. The delta design is the one that most people know and remember because this is the one in which most people have memories of flipping. The 2F1R or tadpole configuration on the other hand can be built much more stable than the delta version and similar or in some respects better than a modern 4 wheeled car.
Not only does the tadpole version cut size, weight, and costs it also increases vehicle efficiency in many ways. One of the more noticeable ways it increases efficiency is with the term (PMOI) Polar Moment of Inertia. A 3 wheeled vehicle has dramatically reduced PMOI which creates a more sport car handling feel. It also takes less horsepower or energy watts to get it moving due to the decreased weight of rotational mass inherent in the 3 wheeled design.
If your goal is to build or design a highly efficient electric car than an ideal candidate is a single powered rear wheel! One that is fixed and does not steer that is
keep the steering up front I concur with others its safer and far less complex.
However despite my personal opinions I am enjoying reading about the Urbee and the work being down by them on the single rear wheel steering. I think they are doing a fine job thus far taking on a difficult engineering design approach. It would be a dream to park it I bet!