[Tuneintosando]

Having a chance wouldn't be so bad, that way it could be used as a desperation move to get a melee user off of you giving you breathing room for one turn.
You'd hit them do damage and if lucky take their momentum a couple of points, allowing you to get two attacks vs their one if they choose to stay or create a gap for the coming turns.
Then if the gamble didn't pay off, you'd still deal damage but not as much as if you'd casted two spells.
your next move would most likely be to run since he'd out dps you in melee if you're both equally well built and leveled.

Now the next issue would be balancing the chance making it a fair enough to be viable risk but not so high that it'd work out the majority of the time.