I use a credit union also, but prior to that I used a Big Bank and it had the same limit on the number of withdrawls on a money market account.
I can do as many electronic transfers from my money market to my checking account as I want, but I can only make 6 cash withdrawls and/or write 6 checks on it a month. So I always write checks out of my checking account and transfer the money from the MM account afterward.
I remember being made aware of this limit back in the mid-80's when I opened the account at the Big Bank, so it has existed for a long time. These used to be called "NOW" accounts (Negotiable Order of Withdrawl).
A money market account is not a really a checking account, so it shouldn't be used as such.