Who positions their speakers straight ahead (zero tow in) and why.

In both cases I ended up with tweeters on the inside and zero toe in. I like the more immersive soundstage I get with zero toe in. I sacrifice a little of the lock in for the center image.

This can work fine with some speakers if there's no side walls. Side walls cause early reflections which are interpreted by the ear as harshness. If the reflection is long (+10ms) then the ear can interpret the reflection as echo location information which can make the sound stage image more palpable.

So in a smaller room where the speakers are near walls you'll want to toe in to reduce side wall reflections- that will make the system more relaxed. If this makes the tweeter too bright, you might have to toe in more so the speakers 'cross' in front of you rather than at the listening position.

In big rooms yes, in small rooms you’d be listening mostly to side wall reflections. 

Reference 3a decapo i has a Deapollito design where the tweeters in this two way design are offset to the left and right of the woofer.This means that time allign correction/ image is best when straight ahead.I tried toeing in slightly and it collapsed the width of the image so I keep them straight on as recommended.

I have Vandersteen 5A's......I have them 1/2 way between straight on and facing me when listening.   I experimented years ago ....I presume that my final setup is correct....the speakers are way too heavy to move.

To many variables to consider on this subject.  Your ears must be the judge.  Sometimes, if your loudspeakers are too far apart - some small toe-in will help bring the center image into focus. This is a simple - no cost variable that we all face with box loudspeakers.