What the A20 is missing from a security perspective is secure boot procedure. There is some primitive support but not really functioning. Some of you may think I am crazy speaking about secure boot, but properly used it is a very strong tool for ensuring that the installed software is not tampered with by untrusted parties.
How serious is such a threat?
I mean I see the point in theory, but in practice the risk of the user losing control of his device because of such a "trusted boot" seems to be far higher than the risks linked to the absence of such a mechanism.
Stefan
PS: by the way, if you boot from the µSD card, you could probably get the same result as a trusted boot by using your own µSD when booting and making sure this card is read-only (e.g. by taking it out after the boot is over).