Of course the harm is the addition paperwork and record keeping required
What record keeping? Cost basis is either average (requiring no record keeping - all done by Fidelity) or by individual lot (records also kept by Fidelity).
You can’t use average cost for common stock except when it is part of a dividend reinvestment program.
It is your responsibility to maintain adequate historical records, not the broker. There are circumstances where the broker’s records are not accurate and the broker has no obligation to adjust the cost basis to the correct amount. (There are some gaps in the actual law and regs such as the receipt of shares as part of the total liquidation of a partnership interest.)
Ira