Aesthetic issues:
One of the complaints against using ReaScripts for MIDI editing is that the scripts do not look and feel as "elegant" as REAPER's native mouse modifier actions. (E.g.
Reaper for Orchestral work and
Display CC event through Curves instead of Bars.)
Practical issues:
It is not always obvious to the user that a deferred script is running (or
which script is running), particularly if the script is not linked to a toolbar button. (E.g.
CC stretching script.)
I suggest that both of these issues could be ameliorated if scripts could change the mouse cursor in REAPER's main UI, similar to how scripts can change the mouse cursor in its own GUI using gfx.setcursor.