This dropped off a named Silverflank Guardian (ape) in LTR, and I'm glad you're enjoying it Jolea! =P
As for rarity, this guy was up when we moved into the LTR and dropped it, that's the only time I have seen him up so I cant give an accurate statement.
I just traded Frostbringer for this item on Emarr server, and I certainly dont rue the trade, it used to be proc then jolt proc then jolt be oom if I didnt have a clarity on, now that I have this am really happy and using epic so its plenty fast.
MoSS = 19/29 Ratio = 1.526 This = 13/23 Ratio = 1.769
MoSS = Sells for about 500 plat, usable at any level This = Probably doesn't sell at all right now, must be 51+
You even get a better mana bonus, at least by the time you can even use this thing. 30 hp is ok-ish, but I really doubt it'd make it worth the loss in damage, when you consider raid monsters hit for way more than that.
It's a good offhand weapon for dual wielding classes, but... no way it's a "good alternative" to a MoSS.