FileMaker has for the longest time had the notion of working with the found set. But in many modern applications for example like iTunes or excel, one can make multiple selections and act on the selected items independendently of what the entier set of data that you are looking at.
I think it would be great to have a way to have a native OS selection hightlight and a way to do multiple selections in different windows.
Then having a script be able to act on those multiple selections would be ideal.
I'm not sure you understand the underlying philosophy of FileMaker. You can in fact make a multiple selection function as you described. Pretty easily using a short script and global variables. But for FMI to bake that into the program would take away from the flexibility of the application. I wouldn't want that.