Its a funny build !

Only played Miss Fortune twice and this build helped me get 10/1/12 they surrender:D

Like I said, they're dead in 2-4 shots....there is no multiple ultimates or cc's. The enemy team will always focus you anyway once they realize you're much stronger than they are, so you're talking about negating 1 ULT every few minutes to give up 600 damage per second. Your primary job as MF is to deliver ridiculous dmg and let your team push; no way you'd trade a few hitpoints and a little armor for that.

Now don't get me wrong; people do it all of the time, but they are not dominating games anywhere near the way they should be. I just now played with one of the worst random teams ever...we were down to the two inner turrets and the other team was ahead 55-24. Eleven straight kills later as I ran down lane pushers and ganked group battles from behind, we dropped their nexus. I was 19/10/17 (the rest of my team was 8/47/21)...but that's against people who started out like 9/0/5 and way, way ahead of me. This build literally gave us a win in an un-winnable game. Trust me....you want 600 dmg per second over blocking one shot.

But its about the spell shield which can help to blow of the first few Ultimates and CC. If your stunned and focused you cant do anything and you'll just die no matter how much lifesteal you have...

You didn't read the guide at all....why add 50 defense when a single item can give you +200-300 offense per half second? I'd much rather kill quicker (and take an extra 150 health per second) than rely on taking an extra hit or two from a zerg. That armor will not save you 5v2 but killing enemies quicker might...and 1 on 1 isn't even a question with this build anyway...they're dead in 2-4 shots.

You should probably add a defensive item in like Banshees Veil for late game or at least consider Mercury Treads.