This is not a new idea, but Doug just blogged about it because Roleplayrescue did first. So, here I am, too.
You can see the concept explained here:
The Defence Roll Reversal
Short version - roll Active Defense rolls first. This makes the defender commit, and means any attack you choose to defend against has a cost (in cumulative defense penalties as applicable, or use of Retreat, or cost in FP for defensive spells.)
Will it work? I think so. From the post, it clearly works for this group.
I don't ever plan on using it, myself.
In general, I don't like increasing the number of rolls. Defense first adds to the number of rolls - because a critical hit can bypass even a successful defense. So you always need to roll for a defense, unless you choose to just take the hit. If you use the usual process, the number of rolls needed for defenses is limited by the number of hits - you don't roll if the attacker critical hits, misses, or critically misses.
Also, I'm not fond of the idea of a critical defense causing a critical miss before we even know if the attacker would have hit an undefending opponent. I find that really messes with how I picture combat flowing.
Pretty much, that's it. We have so much going on in terms of dice rolls with a large group, and a large number of foes. Adding more die rolls, even in the name of tension, means significantly longer combats. I spend a lot of time minimizing die rolls, and cutting down die rolls which don't really need to be there.
Still, read the post. It's clear this is working. As you can see from the post, the players like it. It works in Actual Play. Just for me, it comes with baggage (extra rolls!) I'd rather do without, and a change of perception about how combat flows that I'd rather not have to get everyone onto the same page with. Remember, just because you don't like a rule it doesn't mean it doesn't work, and just because it works it doesn't mean you have to like it.