Not all of us are against it.
@ Becq - Every shift needs to be "paid for". If you've got 23 shifts of stress and consequences, a declaration, and a fate point with an aspect to invoke you get a maximum of 27 shifts in the spell. Trying to go out with a 50 shift spell means 23 shifts are fallout which a) reduces the power of the spell and b) is up to the GM to narrate. Whatever the narration, it shouldn't be beneficial to the caster's intentions. If I thought abuse was intentional, I'd probably have the fallout disrupting the controlled portion of the spell as an unintentional block.
Not at all! Let's say I decide to cast that 5000 shift spell. I have Conviction 4, so I'm going to take a 4997 mental stress for casting it. I can reduce that by as much as 2+4+6+8=20 by taking consequences .. but why bother? I'll just take myself out and leave the consequences for later. Now, in my opinion the spellcasting sequence stops right there; the power-gathering step failed. (If it was important, only 8 shifts were gathered, because the 8th shift was what edged the stress into the non-existent 5th box.)
But under discussion is the case in which we decide the spell goes off regardless. So I've been taken out, but there's still a 5000 shift spell in the air, with the universe somehow conspiring to gather the remaining 4992 shifts for me. (Thanks, universe!) Ok, now I have to control it. I'm unconcious, so I shouldn't be able to control anything, but we're saying that I can. So I roll my Discipline 5 and get extraordinarily lucky! ++++! That means I control 9 shifts, leaving 4991 uncontrolled.
Now I need to split that between backlash and fallout. But if I put it into fallout, that would (a) nuke my friends, most likely, and (b) reduce the strength of the spell. So, what the heck, I'll take it all as backlash. That's 4991 stress worth of backlash, and I decide again not to take consequences, since I can choose that. Whoops, looks like I'm taken out. Again. Still? Whatever; my target gets hit by my 5000 shift spell, perfectly focused on him and with no side effects.
How does this sort of silliness make for a good story? I just don't see it. My read would be that the hypothetical character tried to cast the 5000 shift spell, passed out (very quickly) from the attempt to gather so much power, with the end result that the power that *was* gathered (ie, the amount that would have created an overflow into a nonexistent stress box) would have then been released as if it was entirely uncontrolled. Which *could* actually play a role in a story, as the character basically converts some or all of his remaining consequences into an uncontrolled "fallout" spell (in the above example, it would be from 8 to 28 shifts of fallout, depending on the number of consequences taken). I could see this as a great last-ditch way of escaping from a mook-powered ambush.