This job trait is useless on the jobs that currently have it. DRK enjoys meleeing for damage; casting instead of meleeing produces inferior damage, inferior tp gain AND costs MP. SCH and BLM don't really need TP for much of anything.
However, give this trait to BLU and it would be useful. Gamebreaking? Doubtful. However, of all jobs that could use the trait in its current form, BLU would make good use of it. At the moment, it's dead weight.
EDIT: Though obviously you'd have to adjust it to allow it to gain TP from Blue Magic spells <_<