Visually comparing various map frames, symbologies and layouts require having them open at the same time. I cannot count how many times in a week I have 2-3+ projects open at once. Dragging and dropping (copy/paste) between open projects is a major perk and time saver of ArcMap. This is needed especially during the migration process to ArcGIS Pro when examining the subtle differences of importing items over from a traditional ArcMap user base and recreating templates.
Please enable a desktop user with a named account linked to an org account to open as many ArcGIS Pro instances as their workstation can handle. Being stuck to one instance at a time is a big problem.