is it beneficial?
Yes, because in a 1 disk system you can have more than the usual number of slices (> 5 on a root disk), and in a 2 disk system they can all be mirrored.
Also, adding space is as easy as
metattach d5 10g # add 5Gb to d5
growfs -M /opt /dev/md/rdsk/d5
I am currently setting...