I'm maxed out, and maxed out last year too.
If ones investments are in leveraged instruments like calls and futures, then after levering up to sensible levels of volatility (the Kelly Criterion implies there is a maximum level for ones bankroll and investments, no matter how high ones risk tolerance), one will still have a lot of cash left over that needs to be parked somewhere that at least keeps up with inflation.
I-bonds are attractive for this role because of the retroactive effects of recent inflation. But the cap means it's not enough for all of my excess cash. One who is below that cap might still want to keep a portion in something more liquid. In my case, it's a small enough fraction (because programmers are paid well in America) that I'm not too concerned about the lack of liquidity in the first year.