The current idea is that raising the proc rate on mage from 8% to ~15% (effectively 2x the proc chances as current, though it's not quite the same thing) would be fairly sufficient to address the problem of balancing total proc rates vs JAs.
If, instead, enspells could proc, we'd have to look at the frequency at which enspells could be applied. Best (or worst, depending on perspective) case: Kraken Club with haste/Marches (~65% haste, depending on the brd), which is about 4 hits every 92 delay, or one hit every 23 delay. [Edit: scratch samba, as that would conflict with enspells; duh]
In other words, going from 1 attempt per 4 seconds to 2.5 attempts per 1 second, or 10 times as many chances. Frankly, that would be broken.
One could perhaps limit it to Enspell II's (so only one chance per round), though that obviously restricts it solely to main job rdm, and you'd still have a chance of up to about 1 swing per second using Joyeuse. Only a 4x increase in the number of chances to proc, but still likely considered unbalanced.
So to balance against those extremes while using enspells you'd have to lower the proc rate further, which makes magic procs for non-enspells even worse unless there's a way to separate out the 'type' of magic used to proc (which may certainly be the case, since enspells have 0% chance right now).
And even assuming you *do* separate out enspell proc rate chances, and make use of normal enspells, you still have about a 3x range in the rate of proc attempts that can be made (varying from basic haste to max, depending on buffs, as well as DA/TA/etc). JAs have fixed timers which restrict the rate; WSs are limited by TP building and how quickly you kill the mob; magic doesn't have either of those, which means you have to use a proc rate value that's suitable for a wide range of potential factors. If you choose a rate for the best case, then anyone without a Kraken Club is screwed over.

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