Only thing i would want the constant for is to known what the default divisor is. The default just happens to be 1 (no divisor/off).
However (while unlikely) a new version of lucene could default to using a real divisor (maybe once everyone is on solid state disks, a higher divisor will result in the same speed of access, with less memory use), at which point, if i upgrade to a new version of lucene, i want to inherit that changed setting (as the default was selected by people that probably know better than me what will better server the general use of lucene in terms of memory and performance)
right now, if i want to inherit the default i would have to do a conditional IndexReader.open() and store my setting as a pair (enabled/disabled, divisor), which could be encoded in an Integer object (null = disabled/use lucene default)
if the constant is made public, its easier for me to inherit that default setting.
of course at the end of the day, either approach will only be about 5 lines of code difference, so again, i don't really care too much about the outcome of this
By the way, if you use a final constant, without recompiling it would never change,...
I never drop a new lucene in without recompiling (so that doesn't cause any difference for me)