The dev team seems focused on making sure BLM is not mobile in any way, even to the point of halving the potency of Scathe when PvP came out (thus weakening them in PvE as well). I think the most fun I've had in PvP is on BLM, but it's really annoying to try to cast spells on someone, and they simply walk off. Whereas a BRD can continue to chase you going full tilt (or even a MNK with Fists of Wind), BLM can't really do much if someone decides to leave the battle. Basically, I can just chase them and Scathe repeatedly, forever. For very low damage. This makes it especially annoying to try to kill healers. Between walking off and using line of sight, any competent healer is unlikely to be killed by a BLM of any caliber. It doesn't help much that BLMs are probably the #2 target, right after healers.
I guess I really just don't understand why the dev team feels so strongly about BLM being "rooted" and not being able to cast while moving. I think at least Blizzard and Fire should be able to be cast while moving. Obviously Flare shouldn't be mobile, and I'd be fine with Blizzard III and Fire III not being mobile either, but it would sure be nice if you could cast Blizzard and Fire without having to stop.
Between having to stay still and being easily interrupted any time you're touched, BLM is essentially useless if you're being focused and/or don't have a lot of support from a healer. All things being equal, it seems like pretty much any other job has the advantage over BLM in a 1v1 situation, including healers.
In "other games", it's more the norm to be able to be mobile while casting (at least light spells) and instead of being interrupted, have your cast bar pushed back each time you're hit. Why can't this be the case here as well?
Pretty much every other dps job can do good burst damage without ANY fear of interruption, even SMN (thanks to Energy Drain and Ruin II spam).
I'm not saying BLM is useless or anything, or that you can't succeed in PvP with it, but the way it's designed doesn't exactly lend well to having fun in PvP. Unless your idea of fun is spamming Sleep and hoping it messes with everyone. It's easy for players to be bad at BLM in PvE, and even easier for them to be bad at PvP. I think BLM can remain balanced with other jobs and still be able to walk around while casting Blizzard and Fire (tier I versions only).