Portage should notice and report conflicting USE flag declarations in /etc/portage/package.use/*.
grub: sys-boot/grub -theme
misc: sys-boot/grub theme
Instead of rolling the random number dice, it should notice the conflict and emit a diagnostic.
A similiar check on the portage tree's profiles may also prove useful as a QA test. Different levels can override each other, but there's provable ambiguity if it's in the same layer.
IMHO, detection of such a blatant contradiction should be an error.
It would be valid to do something like this for quick local overrides though. I'm not sure we're that bothered about this.
I feel like it'd be a lot of effort to implement with very little gain, but I suppose the confusion isn't great when hitting this problem?