Having class unloading off by default for CMS has been causing perm gen
out-of-memory exceptions. It is often a surprise to users. It was originally set
to off to reduce the remark pause time. With removal of perm gen the
result of not doing class unloading for CMS can be the consumption of
unreasonable amounts of native memory. Also with the remark phase being
done in parallel know, there is less benefit to having it off.