I know we have done this with point clouds and I would be interested to see how to approach it for mosaicking images.
Point clouds are vectors/drawings and images are, well, rasters/images. Copy/Paste was a very big deal recently added within 18.104.22.168 for drawings and labels, the relevant notes being:
The map window supports copying and pasting data for drawings and labels. Pasting records between drawings and labels automatically maps fields for geometry and / or text to each other.
Pasting records into a table / drawing / labels automatically converts coordinate systems for geometry fields. The conversion is performed only if both the target and source fields have a valid coordinate system (displayed in black instead of red in the coordinate system pickers, eg, in the Component pane).
So, for now drawings and labels are copy/paste in map windows but not yet images. With images, you can paste tiles into tables and use SQL and otherwise manually work it, but it's not the same as copy/paste within windows for images in a way analogous to what was introduced above for drawings and labels.
There are many issues for images that are wildly different than for vectors, more decisions to make, etc., so that's why the intricacy of the manual steps one now must do if that is desired. Automating all that is much easier after pending additions will be merged into the system so that's why copy/paste for images is coming later.
I agree that fun with contours and tracing is an immediate stimulus to wanting to merge big DEMs into one. It seems like every time you want to contour a DEM for a particular region that it will be exactly bisected by two different data sets you have to download from USGS or whatever. :-)
For now, though, the priority is leaning more towards finishing out all forms of vector manipulation/editing/presentation, with building out all raster capabilities coming a bit later. There are, of course, new raster capabilities like contours and tracing being added (got to have some fun with rasters, after all...) , but the full-bore fun with rasters stuff like seamless copy/paste, pixel level selections and editing and so on will come in the weeks and months ahead.
Have patience and, as always, if you have special preferences for something to be prioritized ahead, speak up with a Suggestion.