I don't really know if casters are balanced or not, but I can tell you they aren't particularly fun to play in PvP (for me). While I mostly do BLM or SMN in PvE, I now pretty much universally go WAR to PvP. I feel like I have some kind of purpose, and won't just instantly die all the time. If you could "guard" your BLMs (your team blocks other teams from getting to your BLMs to begin with), I guess it would be better. But since any hit interrupts, and you can't cast basically anything while moving, you're like this little beacon that everyone wants to snuff out almost as bad as the healers. If you go on the move and try to escape basically any dps as a BLM, well, you're dead. What are you going to do? Scathe that guy? Yeah ok. Nevermind how annoying it is that Scathe was nerfed in PvE because they didn't want it to be too strong in PvE, thus lowering BLM's mobile dps even further, and you've got a real winner going on there. Situationally I'm sure BLMs are amazing, but I'm not sure how many people at this point are willing to let BLMs run around and do whatever they please. More than likely the melee dps are going to jump you and that's the end of it.

For Frontlines, where it's supposed to be super casual and fun, I don't see your average BLM or SMN being enticed into playing. Honestly, it's been a long time since casters seemed very effective in FL. I've been doing a lot of matches this past week after a long break, and I've never seen a caster significantly impact the flow of the fights. Usually it has come down to, who can obliterate the other group's healers first (assuming roughly even numbers for each group), which typically amounts to tanks harassing the healers while melee dps bury them (or the melee dps bury their ranged guys while the healers run for their lives from the tanks - later bards/black mages, no heals for you).

For such a movement heavy game, going double or triple for PvP, it's hilarious to see a particular job (BLM) so severely hindered by movement as to be pretty much ineffective without support (other players stunning people running for the BLM, or tons of shiny healers to distract attention from the BLM). Certainly if someone makes a concerted effort to run from you, a BLM isn't killing anything period. Even my WAR can run someone down if I choose to. No one is worried about your Scathe barrage. No one.

Speaking of no one, no one seems to be able to offer a good answer to either of these questions:

1) Why can't BLM be allowed to move while casting Fire 1 or Blizzard 1? Sure, root them for Fire 2/3 and Blizzard 2/3, but you can't even cast the most basic spell on the move? Meh.

2) Why isn't cast time increased instead of interrupted when being hit? FFXIV tries to copy everything else from WoW, even down to the cash shop - why not this too?