I have more feed based importers in the pipe, and i am under the impression that the average arcade admin prefers to suck up as many games as possible instead of carefully picking them individually (barring the general tags at the top etc).
By unchecking everything by default, the average arcade admin must now re-check the checkall at the top, and lose the automatic un-check of already-installed games.
I think that accidentally installing games is uncommon, since you need to have scrolled down and observe all the checked games, ample time to stop yourself. not to mention the average admin probably doesnt care too strongly in that case, and you can still uninstall them.
So, why harm the common scenario admin (installing games with abandon) to help out the uncommon scenario admin (accidentally installing games)? Force the uncommon admin to do the extra click instead, to get to their desired state (everything unchecked by unchecking the checkall).
I cant add a poll easily to this thread, so if anyone wants to disagree with my interpretation, please post it....