Really off the wall job combinations are the best part about subjobs for me, personally. When all jobs become the same, or there becomes a meta, everything becomes repetitive. There becomes one way to clear a dungeon, one way to play a job, one combo set or "rotation". That's so incredibly boring and predictable.

It's far more interesting to see variety. And subjobs lead for so many endless combinations of skillsets. I think I'd rather not be invited to parties by meta-minded gamers with little patience for things outside of established strategies than put up with dull repetition.
During the earlier days of Final Fantasy XI, I liked to play Ninja with Red Mage as a subjob. Was it effective? No, because it didn't get access to either the better White Mage or Black Mage spells. But I liked the fact that it made me feel like a Ninja Red Mage. Some people told me I wasn't useful for their party, so I missed some invites in Kazham. Or was told to go play a White Mage at times, rather than Red Mage. But in the end, I had fun despite those negative and discouraging experiences.

If XIV let me do similar, I'd probably like to try something like Samurai with Red Mage as a subjob. Using a katana and being able to cast Verraise would be awesome.