Broker-dealers often have a short holding period anyway, so the use of MTM accounting is basically a wash and I imagine it's a lot easier than tracking exactly

Broker-dealers often have a short holding period anyway, so the use of MTM accounting is basically a wash and I imagine it's a lot easier than tracking exactly when you bought each security. Things that banks and financial companies hold for a long time get a lot more care in terms of their accounting, in general.