As a minimum, it should be there - and in that very basic sense it doesn't matter whether it's on demand (eg Aperture smart albums can query "status = missing") or real time as it was before. But when you think about it, the on demand approach depends on the user being either sufficiently intelligent or aware/awake enough to notice he/she's done something stupid in Finder/Explorer. Real time's value is that it 's always there and acts as an alarm.
I do question if processing demands are really a valid reason for chopping this feature. After all, do a Synchronize on a big drive - how long does it take to tell you how many files are going to need importing? Not long, is it? Comparing a list of filenames against the SQL and incrementing the missing files counter is a smaller task.
But let's accept processing demands are indeed excessive (even by comparison with preview rendering). So you quickly come back to addressing what's meant by real time, and there's an obvious third way - near real time. Run missing files every x minutes, or as a background process. Don't delete a valuable control.
John