From a thousand foot view, sure there are balance problems with the TP mechanic.

From a practical perspective, it's like this:

1. Fights never last for 15 minutes without mechanics disengages where you sit idle regenerating TP without spending it.
2. DRGs and MNKs in particular, with BRDs sortof, are "bad" at AOEs in the sense that they cannot sustain AOE DPS for longer fights or phases.
3. BLMs and SMNs are "good" at AOEs in that they can sustain AOE DPS without damaging their single target DPS in longer fights.

Shrug?

Edit: In other words the imbalances don't matter a whole heck of a lot until you hit specific content constraints that will more or less break TP-reliant DPS classes -- specifically very long fights with a lot of high-throughput AOE damage requirements (e.g. everyone has to AOE).