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

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.

I have a pair of vintage Polk SDA 1C's, and I position them with zero toe-in because the manual recommends it. I think it's also proper given the interconnect cable for the extra stereo spread feature.