I will clarify concerning Fidelity's CMA.
I just transitioned to this setup and, for me, it is optimal in many ways.
First, it's a one stop shop. ALL my money is in one place. So rather than having my bank, another place for investments, another place for savings, its just all in one place. So easy.
Second, the CMA(cash management account) is currently yielding ~2.1% on FDIC insured cash. Free check writing, free ATM, ATM fee reimbursement, free bill pay, etc.
I'm getting 4.14% safe return in their SPRXX money market fund on my savings.
Optimal and couldn't be easier. IMO.