+1. I've just purchased a PM+ license and have come across the same limitation. The catalogue feature is great for browsing thumbnails when you are offline, but... without the ability to update metadata it has very limited functionality until your media is online again, which can be difficult if the media is spread across multiple storage devices.
My use case would normally be when large amounts of metadata need to be updated in a manual/time consuming way, that I would rather not have to be sat next to my RAID to complete. e.g. going back through a catalogue and updating Geo location for images, or going through and inputting 'Persons shown' data.
I think a healthy implementation would allow the editing of IPTC, colour tags and ratings while offline, and then (ideally) sync when the media came online, or if that's not possible, via a menu command.