I think you're confusing practical and actual casting times. It's rough getting 30 casts per minute even with chainspell or maxed nightingale because of animation delays and the annoying way the game only accepts one command every half second or so. Even spamming the fastest spells and assuming no obligation to any other task, you're probably looking at about 15 casts per minute.
You're also only looking at the DNC's native abilities. Most jobs which go /dnc to dynamis have native JAs of their own they can use which adds to the 7 JAs per minute, war and cor being the 2 best I can think of right now.
There's also a primary difference between JAs and spells: JAs have no casting time. You can do other stuff while waiting for JAs to recharge. You can't do squat while spamming threnodies.
Overall, magic is the most screwed of the 3 proc types right now. To be fair, balancing the procs is a tricky topic though. I'd do it by making it so JAs with a longer recast timer have an increased chance to proc, spells with a longer casting timer have an increased chance to proc, spells matching the day's current element have an increased chance to proc, AoE spells and WSs have a decreased chance to proc on anything besides the main target, WSs with a matching SC alignment to the current day have an increased chance to proc, and make skillchains and magic bursts have a chance to proc any mob. But that's just me rambling again.