I don't understand the inner workings of iCloud or PhotoStream. What I do understand is that every photo from Aperture (and maybe iPhoto) that is pending sync with PhotoStream is duplicated to a folder in the User Library directory. That means if I, say accidentally, mark 3000 photos for sync to PhotoStream, then I waste 83GB of a 250GB SSD.
Of course, this happened on the same day that John Siracusa talked at length about the evils of filling up a boot volume. Guess what I did? Yup. Filled my boot volume with PhotoStream duplicates until the OS made me kill apps. Fun evening.
Luckily, it was also the same day David and Katie talked about Daisy Disk. I've often scoffed at disk space analyzers. "I'm a power user. I know where everything is." Well, I'm here to say Daisy Disk identified this problem much faster than I would have at the command line. When my boot volume is almost full, I don't want to mess around. Totally worth $10.1