I'd prefer a DoT, but this is also a possibility.
A couple thoughts...
Minor Mangle would help with HP tanks, arguably sorcs' biggest weakness. It would help to a lesser extent with your average HP pool players, that's fine.
The thing about this debuff is the timing. For it to contribute to a burst combo, it has to be applied first, before dealing any damage. I am having Curse-Fury-Meteor-Cage-Frag in mind. The intitial Fury impact would probably eat the pseudo-damage Mangle provides. Also, if it gets applied when Cage impacts, it'll be useless for the burst. I would suggest applying the debuff upon cast therefore. That would make the debuff undodgable, though, for better or worse.
Mangle still doesn't apply real pressure. Sorcs desperately need a good DoT. Putting this on Rune Cage would make it appealing to use over Master Reach. I can't see people dropping their Master staves for Minor Mangle. This is the biggest contra point I have.