As the others said, it doesn't remove the limit so much as extend it, and let you do some more with it.
I figure, there's a couple ways to do it. Let's call a given caster's effective conviction X and the effective discipline Y, and assume they're the same. Ergo, the most power that can be put into one spell without incurring a consequence is X+3.
So say you want a Weapon:X spell, and you want it to last three extra rounds. That means you're taking a 4 shift mental hit, plus backlash if you can't roll to Y+3 as well. If the spell succeeds, you still need to roll at least Y to keep hold on the power--with three extra rounds, assuming Y=X, at least one of those rolls is going to be less than Y, meaning you're taking some more backlash. And each time you take backlash, it constrains you further--if you end up taking two physical backlash hits, counting the initial backlash for casting, anything that lays a hand on you's going to force a consequence.
Alternatively, you could go with a Weapon:X-3 spell to be perfectly safe--much less chance of having to take backlash (I figure you want it at least 2 shifts below X to be safe on the roll, 3 to be really safe), but at the cost of a much less effective spell. You might finish up the four rounds of casting without any stress besides the initial mental hit, but the spells might not have enough kick to do real damage at the same time.
Plus, in both cases, the wizard's almost certainly going to be attacked in turn, which might force a change in tactics.
It allows for longevity, yes, but only really effective longevity if you're willing to deal with a spell that's significantly less powerful than you'd usually cast, and only if you're willing and able to do nothing but attack, attack, attack the whole fight--useful in some situations, but impractical for others.
I honestly don't see the appeal of this. Yes, it would probably work as you proposed, but why would you want to do a spell like that? If you have to roll for the spell again, why not simply do a new spell? I'm pretty sure you'll get more firepower out like that. Even if you want to use the spell against multiple weak targets, I would rather split the spell to attack multiple targets at once than do the extended spell you proposed.
To extend the duration of the spell, you are either going over your conviction safety limit, increasing the shifts of casting stress, or you are limiting the spells power, making it a whole lot less effective than it could be.
Well, if you're fighting a bunch of badguys who have decent dodge rolls (like the +3 for the vampires in the example), a spray attack might have little chance of hitting more than one of them. The way I figure it, given the odds, you want at least a +1 over the target's dodging skill to ensure a hit--so Harry in the example isn't going to hit more than one vampire at a time unless he's got fate points to spend and is really lucky on the roll. But he can hit each individually.