I voted this up and added the following comment....duplicated/reposting here for maybe more exposure....turned into a semi-editorial from me...
I spend most of my time in ArcCatalog developing tools that aren't tied to a project, user, or machine. My preferred end product are python add-ins toolbars/buttons, which to my knowledge are not, and will/can not be supported in Pro due to ribbon UI (although the toolbox can be accessed and used).
My use case is being able to manipulate/analysis/geoprocess data, including maps via arcpy.mapping. I know that toolboxes can still be accessed, and "tasks" are the new workflow model, but an easy interface with the ability to develop/manage data and maps programmatically is why I like and use ArcCatalog. I don't have to create a project for this, just open it up and develop, view and/or manage all my data and tools. I know ArcCatalog will be around for quite a while and so this development/management workflow will continue for my use.
On the up side for Pro, I know it does have some nice features, and running my tools written thru ArcCatalog run much faster, as long as I make them compatible with python 3.x, when necessary. Pro is solving many of the issues that many users have been asking for, for years (multi-layout, real 64-bit, etc), so that is a plus. Also, I know that trying to build some of these requests into the Desktop platform wasn't possible, practical, or cost effective -- and having them try would have caused more of an uproar from all of us, as new issues, time delays, and rising costs would most likely have happened. So, the approach of starting clean, from the ground up, and adding features in as it grows is probably a good approach, and less disruptive to those of us that are long time users of Desktop (as much as I and many others don't want to admit sometimes). It is after all still in the 1.x version (anyone else ever remember a 1.x version of any software being a completely polished product? MS Windows 1.x,). Sorry about the editorial, but I have hope for the product and ideas like this getting serious consideration and hopefully implementation.
So, for the near, and still semi-distant future, I will continue to rely more heavily on Desktop, while playing with Pro when I have time. With each release more functionality is coming out, so I'll hold judgement on 1.4+, but the "simplicity" of quickly opening a Catalog session, doing whatever, and closing down, at least so far, is not there. Also, our shop will be a mixed bag of users and what they use for a long time to come....so will always need to have the data review and management features of Catalog.