I've found that it's balanced only in so far as casters can attack at range and as such lose themselves in a group. I can often get 2-3 spells off on a target that doesn't know exactly who is attacking him. Contrary to Deelo's suggestion though, walls are the bane of casters (at least BLM) in PvP. if you're being hit with fire just duck behind a wall and you're perfectly fine. Almost no way that the BLM will pursue you into close quarters because it's suicidal. I've often found that 'being made' as an attacker while playing BLM means you should cast sleep and attack someone else, retreat, or finish up quickly (swiftcast + flare, but that will no longer be much more powerful than fire I).

I never really took much note of everything interrupting casters because I'm used to XI where, more or less, everything interrupted casters even in PvE. I guess to me it seemed normal.