If you are skipping over enemies or missing collecting the gems they drop, Member's Card may seem of low value. Shops do not spawn on every level, and there is no guaranteed height they appear at in each level when they do spawn. Shops are the only place you can spend gems, and having more charges or health makes all the difference in survival time. Because of this, Member's Card is likely the best upgrade for anyone collecting gems.
Member's Card mitigates the reduced shop spawn chance of using Arm Spin Style.