One of the main purposes of personas is to create a relatable character, one that you can try to empathize with, and which can be easily used in product-related processes in the organization. It will be very difficult to accomplish that using a an abstract construct with just behavior. A persona will need to be given human features - a face, age, gender, name etc., so that a believable character is created.
To what extent this needs to be backed by research data is a good question, especially in cases where this is not very important to the product domain.
In either case in many domains you can accomplish this part post-hoc, e.g. by using social networks to find the sociodemographic characteristics of your personas. Domain-specific networks would be best, although Facebook and Linkedin can also provide great data. It's also common practice to supplement data from professional forums with more personal data from FB, so that you can use a professional forum to find the name for a person who fits your persona, and then form an impression about their sociodemographic details based on their FB profile.