This is a personal opinion, rather than a “management” response.
SOTA launched with the five UK associations. These could all be covered by the OS grid reference system. EI, added later, is also covered by this (on the same grid as GI). For me, the major advantage of the OS system is the fact that you can buy a road atlas that uses this grid as its reference. I use the Philips OS road atlas, which is great for identifying target summits from the OS grid references.
It is true that inconsistency has since arisen with the addition of new associations, but personally, I wouldn’t want to address this by replacing the OS GRs by Lat/Longs in the UK ARMs - for the reasons stated above.
The excellent know-how, and collaboration work done by Martyn and Jon on this matter, is greatly appreciated.