Firstly, there is an innate physical critical hit chance, why can't there be one for magic?
Secondly, Magic Bursting needs some sort of enmity- brought with it. It would be totally worthwhile and incorporated into battle if it shed enmity from the black mage...
If anything, I think skillchain and magic burst damage should not put any enmity on the person who made it. I'm not sure about skillchain damage, sometimes I see melee who open or close the skillchain pulling aggro... but in any event, if there is enmity involved with skillchain damage (not the weaponskill part) then it should also either be enmity shedding or just not supply any new threat.
This suggestion comes from an endgame disappointed character who thinks skillchains and magic bursting are awesome, but nearly forgotten and now only accidental unless solo-skillchaining.... The biggest problem we have with skillchaining and magic bursting at higher levels is the obscene amounts of threat from the additional 1k to 2k~ damage brought from the successful attack.
Please, please, please. Magic Bursting is cool, but lately I don't want to burst more than a tierII spell because my little blackmage booty can't tank very well.