Cropping of digital images assumes fixed final image resolution for a given
print size (say 1795x1205 for 10x15 print at 300 dpi). Framing with the new
rectangular selection tool is visible and convinient, but requires to set 
aspect ratio with high precision (4 digits after dot). 

In such a workflow each photograph requires typing of aspect ratio. But they are
fixed for some print sizes at prefered lab. If user can remember some preferred
aspect ratios in GIMP settings and then either type aspect ratio directly or
select it from a drop down list it would be convinient and quick. 

The drop down list can look like:
1.4896 - 10x15" landscape crop
0.6713 - 10x10" portrait crop
1.4013 - 13x18" landscape crop
and so on. 



Other information:
By the way, fixed aspect in 2.3.7 for Windows crashes GIMP.