Why should this be true? Shouldn't we be allowed to increase the amount of memory the guest has across reboots? That's equivalent to adding another DIMM after power off.
poweroff is equivalent to exiting qemu, not to guest reset.
Not generating tables on reset does limit what we can do in a pretty fundamental way. Even if you can argue it in the short term, I don't think it's viable in the long term.
I don't think so. The procedure for adding/removing non-hotpluggable hardware is: poweroff, plugin/-out hardware (change config in qemu), boot. Hotpluggable hardware doesn't need acpi table updates.