I guess where I was going with this was when I read the $200K cash example above. If you invested $50K in T-Bills, you would still have $150K in cash left where you would earn IB interest on $140K and earn nothing on $10K. In order to earn money on the $10K which is the entire point of this discussion, you would have to buy T-Bills on the entire $200K in cash. If you do this, then yes, you earn extra interest on the $10K, about $165 based on current t-bill rates, but you lose $2,000 in trading power. If you make 8.25% on your trading money, then it's a washout.
For those trding accounts that are being used as sources of income, such as paying out your salarly, paying monthly bills, etc..., I think it would require a bit more work continually monitoring your accounts to make sure you're 100% invested and then selling off exactly enough t-bills to support yourself.
I'm sure there are many accounts where it is worthwhile to earn the extra interest by buying t-bills, but I think in this example, a large cash account supporting underlying investments, buying t-bills does not seem the best way to go.
For those trding accounts that are being used as sources of income, such as paying out your salarly, paying monthly bills, etc..., I think it would require a bit more work continually monitoring your accounts to make sure you're 100% invested and then selling off exactly enough t-bills to support yourself.
I'm sure there are many accounts where it is worthwhile to earn the extra interest by buying t-bills, but I think in this example, a large cash account supporting underlying investments, buying t-bills does not seem the best way to go.