Jei has a nice discussion on that here
A little confused...
Really the best jobs you can use as a sub are the basic jobs.
rdm/blm = more magic power for attack magic, nice MP
rdm/whm = curaga, status curing magic.
rdm/smn = Lots of MP, gets autorefresh when rdm hits 50 / smn25, you can also use carbunkle as a puller.
rdm/nin = a neat fun class however expensive if you start using powder. I hear that this burns out though since in the higher levels you need to be a true casting mage because hits doing do much damage with a rdm.
rdm/drk = A class that gets Drain AND Aspir a whole 10 levels earlier than rdm/blm. That is very usful in my opinion. If you can get aspir at level 40rdm/20drk. Thats a good thing that will server you well. I get jellous when the blms cast it.