Not sure about merely being the holder of a note, but I know the California licensing rules about originating loans are very strict, and non-compliance can have draconian consequences (I vaguely think the loan might be voidable by borrower without payment of interest, or something like that). I'd also think that if California is like most states, you may only be able to use its courts if you are licensed to do business in California, which could be a problem on a foreclosure or guaranty action.