^^ this was my understanding, too. But that's not always going to work as enough incentive, and I agree with the OP: I'd be LOATHE to let a fantastically lucky roll go to waste.
There's also recourse to the general uses for extra shifts: increase speed, precision/accuracy or quality of the action. Say you were trying to trip someone to put a fragile [Prone] aspect on him. With enough extra shifts (5-ish, but I'm still new to this so am erring in the direction of more shifts), I'd say screw the "non-combat only" rule for overflow actions, and let you make a sweep-kick that also puts that [Prone] aspect on his buddy. Or, keeping it to the initial target, throw a [Why Are There Three Of You?!] aspect on him, too, as he bumps his head on the ground. Or, get a free invoke-for-effect of the target's previously-revealed [Desk Jockey] aspect, declaring that a [Bum Knee] was what put him behind that desk. Or, if you're as nutty as I am, give your more socially-inclined ally an Empathy assessment to catch the look of [The Love That Dare Not Speak Its Name] on his buddy's face as he watches your target hit the ground: Your trip reveals that badass gangster to be a closeted homosexual! Oh, what fun!
Let funny/interesting be your guideposts. If the result is that your minions are cleared out faster than you expected, well, there's always another van full o' minions just around the corner (or lean more heavily on compels for a few scenes, or ramp up the stats on the next baddie, etc.).
All that being said, if your players are consistently getting these huge overflows on their rolls, then you probably need to compare their skills to the oppositions' to see where they aren't lining up: I quickly learned that throwing a bunch of brainless thugs at my Skavis with Great Intimidation and ranged Incite Emotion was about as effective as throwing them at a wood chipper.
