> It doesn't make sure that they're worth what you were promised, just that if the broker goes under, whatever they were holding on your behalf gets returned to you.

This could be mitigated substantially by holding short-term treasuries. In theory they could still be worth less than the original principal amount from time to time, but not by very much, and even then they would always be back to at least the original principal amount by the maturity date in e.g. six months.