A Magic Critical appears to be a random boost to nuke damage independent of day/weather bonuses. The effect seems to be a bonus of +10 MAB. By default, a player has a 0% chance of magic critical . So far, the only known way to increase this is by equipping Witch Sash or Wise Strap. see http://bluegartrls.com/forum/showthread.php?t=63710 for more info on research into this
Ironically you'd be wise to NOT use this grip, as 3% chance of MAB+10 over time only equals about MAB+.3 which is comparable to a Bugard Strap +1. If you're solo/duo, most of the time that extra damage will be wasted as overkill anyway.
I'm no expert at mage stuff but Gumytail i'm not sure what you are saying would be correct, i'd say it works like melee crit hit, chances of extra dmg (aka magic crit hit) is +3% so like 3/100 nukes will do a surprisingly higher amount more then usual, but thats just a guess on my part :)
Goetia gloves +2 increase magic critical hit damage by 5 MAB, making this strap worth roughly +.45 MAB, pushing it much closer if not surpassing bugard +1. Genesis locket also increases magic crit damage, although the neck slot is highly situational. I expect we'll see more gear that increases magic crit damage in the future, making this piece more valuable, though by then it's possible that we'll have a strap that surpasses everything else currently available.
Using this alone without any other Magic Crit gear i would agree, yes this isn't the best piece to use.
On the other hand though, people who can afford the Witch Sash which is a bit more expensive than this, you would look at a Magic Crit Rate from 5% instead of 3%, meaning you will crit more than with only the grip.
