For the crit to make IE far stronger. IE and PD is core for any ad carry.

If the AS boost out of stealth is more than enough, then why do you recommend Phantom Dancers early?

You're getting an AS steroid from your stealth. It's more than enough. I happen to play annie. Even if you don't get stunned, you're squishy enough to go from full to 0.

I would recommend twitch to roam :/
I personally usually take bot lane, or top if there's no jungler, I'll start ganking whenever we have a chance.

The vamp scepter is so that you can stay in mid as long as possible without going back for the first time. The Phantom Dancers come last so you can get AD to work in tandem with the lifesteal. Berserker's Greaves are a must for the attack speed boost-I'm surprised you recommend Phantom Dancers and not them. Ghost is so you can move between lanes quickly, and if someone is stronger than you(and by that I mean, if you have fed them), almost any mid is screwed. The effect of armor pen is slight unless you are stacking it, and unless you are against Mord in mid, I doubt you will be facing much armor anyway. The armor runes wont stack well either, and with all the escape mechanisms Twitch has, he will rarely die unless he is stun locked or ganked, in which case, he will probably die with or wothout armor.

Of course Twitch is a glass cannon - like you said, he has four bars of health at the beginning, and it doesn't get much better. I'd love to find a tanky Twitch build, but the concept just doesn't work - if you want a tanky mid, use another champion. I'd love it if my first build worked for every circumstance, but in point of fact, it has worked several times for me very well, and that's really all I can ask.

Thanks for the Annie comment, by the way, I forgot to talk about Cleanse.

You're missing a point at lv 4.This build yet another glass cannon one. You WILL get wrecked at mid against an ap if they have any sense at all. If you're getting an IE why don't you just get the phantom dancer sooner. Capitalize on the crit dmg. Twitch is an AD carry. Runes should always be armor pen red, defensive yellow, defensive blue. The AD quints are ok ish for last hitting. Why a vamp scepter, you have 4 bars of hp lv 1 and if you get zoned out by someone stronger you won't be getting health back at all. Start with shoes and 3 pots or something. Most important thing is, you're too squishy for mid. At lv 6 an annie will kill you full to nothing. Same goes for MOST ap carries. If twitch is "very slow", then get boots of mobility. They work great on stealth champs.