Slice options is required if you are adding .. well slice.

You can add it without slice, but inside ldom you won't be able to use it as zpool vdev.

I personally like using slices, since it gives me much more flexibility. Slice with Solaris SVM for mirroring (if no external storage) and i use MD devices to add to LDOM(s)

Beware tho, Oracle Solaris 11.1 you must have 17.5 patchset if this to work.
They had the bug from 12.5 forward, which rendered SVM devices on slices unusable inside ldom (it didn't boot with strange messages in syslog).

Of course, because of SVM's great flexibility workaround was to break to mirror, boot from one mirror member, add another, and use mirroring inside ldom.

The downside of this is obvious, if an internal disk fails, you will need to replace and mirror inside every ldom that is using mirrored slices, instead of doing it centrally via SVM on control domain.



Hope that helps.
Regards
Peasant.