Adam: 

From your various responses, it seems that you do NOT have a Circle account.  Surely, I am NO Circle expert because I only started using it at the end of September, and on October 1, I transferred $500 into my Circle account (which translated into 1.29263 BTC.  A I type, my previous $500 is valued at $441; however, the amount of BTC has NOT changed, only the dollar equivalency of my BTC holdings.   

I have NOT yet seen any option to hold my Circle account value in fiat rather than holding in BTC - possibly Circle could add that feature later - though I have my doubts that they are going to allow such (b/c the account would convert into a trading account). 

Actually, before I opened up my account with Circle, I was wondering how Circle was going to prevent people from trading with their zero fees policy, and it appears that their outrageously low weekly limits (starting at $2,500 per week non-immediate transfer or $500 per week immediate transfer); effectively make it difficult to engage in any kind of high quantity or high frequency trading, and their weekly limitations also make these accounts very small potatoes...   Even Coinbase has higher weekly and daily limitations than Circle....  My Coinbase allows for $1,000 per week immediate and $50,000 per week, and the Coinbase limits are constantly updating in order to cause less restrictions. 

Note that my Circle account is brand new and there is a disclaimer that my weekly limits may increase with the passage of time - supposedly as I become a more longer term account holder (we will see?).