+1 for the per machine opt-out. I have some older machines that I only use infrequently but still want to use CrashPlan on. It's unlikely any would be turned off for six months, but one of them did have a multi-month disconnect from CrashPlan due to a weird Java version conflict.

(Also, because I tend to keep my older machines and still want to use CrashPlan there, it's harder for me to use the "adopt a machine" method. I bought a new computer and just finished re-uploading about 200GB of near-identical data.)