I use them (midpoint to midpoint aspects) within charts but not between two charts (synastry) -- They do appear to be relevant within a single chart, although I've always considered them minor influences, unless both pairs share both a midpoint and aspect relationship - then it's generally a major configuration, for example the grand cross, which has two oppositions with squaring midpoints.
This is a good reminder to me that I havn't been paying attention to midpoint-midpoint aspects in charts lately! I need to drag out the script I wrote to find them again
I have never attempted to compare a quantity of known synastry pairs for midpoint-midpoint aspects, so I have not actively disproved them as useful, although I would think the influence would be quite minor.
I should also add that 9-10 degrees is an amazingly wide orb for a midpoint, especially considering the number of midpoints in a chart -- 45 between the major objects, so 90 midpoints when you consider near and far! In many charts, that would lead to any degree in the chart being "hot" -- I generally use 1 degree, unless a luminary is involved, 1'30, or 2' for the Sun/Moon midpoint... But I will admit to being more conservative on my orbs than most are.