How can you say 4x5 is not better in image quality than MF. The fact that it has more grains per square inch, gives it better image quality in terms of resolution of detail.
With modern films, Tmax 100 200LPM, very small grain, in 8X10 to 16X20 a 6X9 negative provides enough detail and small grain, sure a 4X5 will have more, but detail that you really cannot see at viewing distance.
4x5 back on 'yer 8x10 

. Discarding a potentially great picture because I don't want to crop the image seems, well, kind of silly to me.