Getting that close means the opponent will get at least 1 or 2 autos plus whatever spell they choose to chuck your way not to mention potentially over-extending for a gank. A 675 range Q is almost free harass as a mana user won't keep up over time.
Your autos + two Ws deal more damage in four seconds than four seconds of their auto attack damage, not that this will really pan out this way (unless they run from you when you QEW in, then you pursue to get the extra W in). You shouldn't be going in there when they have a spell off cooldown to hit you with either and not many champs have an ability on as short of a cooldown as you.
You punish every missed skillshot (or just spell on cooldown) with an EW and the movement speed boost is enough to get you out before taking too much damage (that is reduced anyway). EW is an effective 1075 unit range as well, though just Shunpoing to them is a lot easier to do, even at just the 700 range.
If I go in to EW, the opponent has to react. Most mids can't just sit there and try to trade with Kat auto's and another W up in a few seconds. Especially if I do it after dodging whatever skillshot they have (or trust myself to dodge a reactionary skillshot snare).
Also, what match up we're talking about matters a bit here. There's a few point-and-click champs that I'd be hesitant to approach (say, Kassadin) or some burst champs that are nasty (Annie with stun up, fuck Leblanc in general). Then things like Modekaiser, Kennen, Malzahar can just be rough in general. Pretty much every skillshot champ is fair game though.
I dunno. I'm open to try anything but I hardly think W is a clear choice. I will be trying a magic pen build today though
To be fair, I haven't played leveling Q first in a long time.
Also, free champs this week:
Cho'Gath
Katarina
Viktor
Ziggs
Dr. Mundo
Miss Fortune
Vayne
Skarner
Alistar
Janna
dat Alistar