Yes I know.
Doesn't dmcrypt encrypt all blocks on the partition / disk whether they contain data or not? Ecryptfs encrypts the data only making tools like clonezilla more efficient if you wanted to image the system quickly before an update or for any reason.
Ie a 500gb home partition with 10gb of data will result in a 500gb saved image using dmcrypt and a 10gb saved image using Ecryptfs.
Encryption of the data only is sufficient for a lot of people, I just think it would be good to have that choice.