To throw another spanner in the works, I think the concept of using blood alcohol concentration is not a great measure of how likely someone is to cause an accident. Some people are much better drivers than others. Some are more attentive. Some have better visual judgement. Some have better reflexes. Alcohol affects many of these, as does nicotine (to a much lesser extent, but still), caffeine, other drugs, legal medication, tiredness, mood, stress, peer pressure, and the list goes on.
0.05 is probably a fair limit for a skilled driver, but for many others I'm sure a much lower BAC would yield the same "increased risk".