Claus
As a current Storm 17 owner, I can agree with quite a lot of what you say. I'm sure the extra weight and stability of a BR20 is a boon single handing in more "challenging" conditions, but then isn't that exactly what the concept of BR's is about?.
Unfortunately it is never just about the sailing. Matt made the point about launching in less than ideal conditions, where less weight and easier handling is going to open up lots more possibilities. I would add to that ease of handling on land,less storage space needed, less use of finite materials in construction, lower cost and, exactly as you mentioned, easier towing with small vehicles. If the BR17 is big enough, and that is the crux of the matter, for the type of sailing which a person is comfortable with, such as river/estuary sailing (and here might be an opprtunity for a subtle distinction in the name!),then it is better than the BR20 for that person. The added stability of water ballast compared to alternative craft of a similar size/weight/cost, is going to be a real bonus and a clear distinguishing factor.
As regards the storm17/19, well it might not be totally relevant, but to be complimented frequently about the style and looks of one's craft is always nice.