> I'm sure trading on margin is for some reason actually a positive concept for the economy at large for reasons I can't understand.

Don't underestimate yourself! A lot of times when something seems stupid and socially corrosive, it is. I don't think there is any reason for margin trading other than it makes a few people a lot of money.

With margin trading you don't need to worry about your cash balance. It can be in something like COST or TLT and you can still trade.