Once MJ is legal, people won't need to hide the fact they are purchasing it.

As long as it is not 100% legal especially at the Federal level, banks will be pressured to not provide credit card merchant services to dispensaries even though these dispensaries are legit at the state level. This is where Bitcoin will step in. It may take some time for cannabis to be legalized federally so there is plenty of time to take advantage of this "grey market".