Yep, you are correct... one story that this site doesn't tell is the move set you will be having. That realy just depends on what a person wants out of there sub though. Do you want it to make your main class stronger, or to add moves that will make you a better fighter, or a nice mix of both.
An example of subbing just to enhance main stats is taking Summoner as a sub... it's a very dumb idea move wise, but it gives an extreme boost in MP.
