In reply to Jon Stewart:
> climbers operating in the 6+ grades have relatively similar levels of strength and technique.
debatable...
anyway i'd say grades below 6a don't need a finer subdivision because of other reasons:
1)history. Before some date that must be in the 50s or 60s, bleausards were using a welzembach-like scale, closed at VI or 6 (the limit of human possibilities, probably defined as Pierre Allain's limit...).
Then they started to open it up : 6a, 6b, 6c...but also 6d and 6e and 6f were invented, later rearranged as 7's.
And the reference for fontainebleau, bleau.info, does use the following subdivision: 4+,5-,5,5+,6a,6a+,etc...
2) As one point all climbers get to a point where getting even better/stronger takes such a considerable amount of time/effort that very fine grade subdivisions individually make sense (not only in terms of "+" but even in terms of hard or soft for its grade...).
Probably, most climbers only experience this phenomenon at grades higher than 6a?
3)Grades higher than 6a start to be perceived as having a sort of "status" value, which is an incentive to the finer subdivision.