With this you can encrypt one dataset and leave another unencrypted, on the same block device, paying the cost only or the encrypted one.
ZFS is both filesystem and logical volume abstraction, so to implement different keys on different "filesystems" on ZFS, you'd need to expose them as block devices, not filesystems, and then use your encryption du jour on them, then your filesystem atop that - which also kills most of your compression or encryption properties, since you're doing it before ZFS sees the data, so to speak.
However, I realise this does not quite fit into the current kernel architectures.
The one really nice scenario this opens up that is discussed in the pull request linked in another comment:
1. Server A has an encrypted dataset in pool foo, currently not decrypted
2. A can send full or incremental streams from that dataset to server B
without decrypting
3. B can receive those streams and import them as encrypted
datasets into the pool without decrypting or really ever having
even seen the keys
4. Server A can restore from B as required
5. The owner of the key material can log into B and unlock
the dataset as if it were on A