Why can't you have a looker and a dancer in the same person? Although I get your point, with 9 members not everything has to be perfect. They got Yuri and Yoona, they can have 7 others.
If you check the idol scene in 2007, SNSD was, if not a brand new concept, somehow a new approach of girl groups with so many members. One of the main features was to have a wide range of personalities within the group, to attract as much as possible the target consumers, that was part of the marketing: the dancer, the actress, the model, the cute, the singer, the popular, the funny etc. It just happened (or maybe was on purpose for contrast or sympathy!) one of the girls was somehow the "ugly duckling".