well, it's a bad combo right now because your subjob is half your current job's level.. making the highest a sub can go 35. If you switch it around, it MIGHT be o.k.... but frankly, mixing melee and magic like this is always a horrible idea. Ninja/red mage works but that's about it.
Now you may think "but runeaxe, paladins and dark knights are hybrids, right?" Well, the way those two jobs are set up, they get spells a few levels, just a few, after the black and white mages do. They get spells faster than a melee/magic combo. These combos are good starting out, but later on, their spells are just too weak to be useful, plus you lose benefits of subbing a job that would add to the thief's primary stats, such as a warrior or ninja.