I know everything is YMMV but it's hard to see it as 'the biggest benefit' unless you're a CK and could hang your hat on an upgrade clearing. Even at EXP level it strikes me as a nice to have, unless you're really putting time into game planning your flights for upgrade potential. Especially given the deterioration in cabin service standards on short hops, there's not a huge amount of difference in 8A and 4A on a 738 between MIA and CLT imo.