I guess because I do a lot of backpacking and hiking, I'd rather carry an extra 8x10 holder or two than a 4x5 reducing back and associated film holders.

Also, I have higher quality long lenses for 4x5 around the same focal length of my wide lens for 8x10, so I'd rather use the ones for 4x5. And, I'm not typically doing much long focal length work, so being able to use a long 8x10 lens on 4x5 isn't that much of an issue for me.

As to bellows flare, however, I assume a larger, wider covering lens used with a reducing back will put a whole lot of extra unnecessary light for the smaller format. (Then again, the same light would be in the bellows for the larger film format, too, so I'm not sure).