He says if you upgrade to Mavericks, Aperture will automatically be updated to be in the Mac App Store and do the upgrade to 3.5. He got smarmy when I told him people in the blogs for Aperture were having this done WITHOUT moving to Mavericks.
As I reported later in this thread, the update does appear and I got it (on Mavericks). However there doesn't really seem to be any important change unless for some reason you prefer Apple Maps over Google Maps.
So you aren't really missing anything by not getting the upgrade. If you have a mixed (Mavericks/Mountain Lion) environment you probably don't want to upgrade because the libraries are not backwards compatible.