A side effect of this is that default flags can not be disabled.
The LiveCD should be built with the desktop profile, so it will have all the flags enabled for that profile anyway. The problem with profile selection in the installer is that the installer doesn't have the ability to process the profiles on the fly, and it really doesn't work when using a live tree.
You coul use "emerge --info" to get the default use flags for the snapshot of portage that comes with the LiveCD. Or you could even sync the portage tree before this step, to make sure the list is realy the current one... But syncing should be optional as it can take a very long time...
The installer does use `portageq envvar USE` to get the default USE for the profile that was used to build the LiveCD, which is the same profile that will be used if the user does a GRP install or uses the stage3 tarball.
Syncing the tree beforehand is not an option, since nothing is partitioned at that point and there is nowhere to put it.