"I don’t want the state knowing that I use cannabis. It might effect my job, or my medical insurance rates. Why should I give up my privacy like this in order to be a medical patient?"
As I understand it, there is no legal mandate to register with the state. However, registering with the state will allow you to avoid paying tax. Here is a summary related to cards from CalNorml:
ID CARDS: Qualified patients must possess state ID cards if they want to be exempted from the 7.5+% sales tax currently imposed on marijuana sales (34011(g)). However, they will be subject to a new 15% excise tax and a $9.25/ounce cultivation tax. No card is required to enjoy other legal protections of Prop. 215. The cost of the state patient ID card is limited to $100, or $50 for Medi-Cal patients; free of charge for indigent patients (11362.755); this is a reduction from the prevailing fees in most counties. Identifying information in the ID cards is made subject to the Confidentiality of Medical Information Act (11362.713).