Suppose you were hit by a crypto variant.
Suppose you had great backups -- terabytes of the stuff.
Suppose you wanted to recover the last [week][day][hour](choose one) that hadn't been backed up.
Are there any tools that could exploit your knowledge of 98% of the backed-up encrypted content, to recover the remaining 2%?
... if such a thing were possible, then it might make sense to turn your crypto variant loose on more and more known data, in order to reach that critical mass of known content necessary to recover the key.
Sign me, Crypto Ignorant.