"I went to my local dispensary and handed them my medical card. They wanted to see my letter of recommemdation, not my card. I prefer carrying around my card, why don't they take it?"
In CA, the 'medical card' is not really a legally valid document, unless its issued by the states medical marijuana program. The document dispensaries are looking for is the 'doctors letter of recommendation' which is a legally valid document in CA.
When you get a doctors recommendation, the company issuing the paper will often also provide you with a card to keep in your wallet or purse, but this is really just a convenient representation of your 'doctors letter of recommendation.'
When you visit a dispensary for the first time, they will ask you for the 'doctors letter of recommendation' and will likely not accept the card alone (as per CA state law). Once you are already on file with them, they are likely to accept the card from then on, but your ID will also probably be fine.
If you have any doubt about getting in, its always better to carry the original 'doctors letter of recommendation' with you.
Final note: Although you may elect to apply to the state of CA for an official card issued by the state, this is not a requirement in CA and most people elect not to do it, and prefer to remain off the states database for privacy reasons.
In other states, the laws are different and often do require you to register with the state. In these states, the doctors recommendation alone is often not sufficient to gain access. You should check on your local laws for clarity in states other than CA.
The recommendation letter is the legally recognized document that establishes your right to purchase medical marijuana legally. The photo ID card is merely intended to be a convenient representation of this letter. It is a good idea to keep your letter of recommendation accessible at all times.
To circumvent any issues I have a copy of the letter of recommendation on my phone as a document AND as a jpeg. It can be emailed, faxed, or texted to the dispensary. You can also show it when you are present. When paying a visit, I register in advance and send a copy of my credentials so I don't have to deal with any issues upon arrival.
Each dispensary is different. Just call ahead to ask them what the acceptable form of your recommendation would be. Also, it's best to always carry your letter with you whenever you're transporting cannabis medicine.