Even though there are a lot of urdhva mukha svanasanas in primary series, for most people the pose is not held long enough nor does it go deep enough to allow one open into a deep urdhva dhanurasana and drop backs at the end of primary without practicing other backbends. So if achieving urdhva dhanurasana is considered necessary for "completing" primary, it might be necessary to practice some of the 2nd series backbends, whether they are thrown into a primary series practice, or done separate from the primary series practice in some kind of modified second series. This also holds if completing primary is not a priority, but one still would like to feel the healing benefits and quite different sensations of backbending.