Originally Posted by
cxwrench
You have got to be kidding. Your assumption is that the rider will not look in front of them and make sure the distance covered while looking down at their cassette, which takes about 1 second, isn't occupied by a giant pothole and/or big truck.
It has caught me off guard a few times when doing it in urban traffic conditions. The main problem was drifting ever so slightly to the left or right in that one second glance, almost getting into the path of the fast moving traffic or bumping to the rider beside me. Luckily, none resulted to accident. Nevertheless it's a source of distraction and keeping distractions to a minimum or zero helps make things safer.
Definitely NOT a good thing to do on group rides.
I went 1x and started using helmet mirror for one reason so my eyes are glued to the front view of the road at all times. Sure you can do the same thing with 2x but 1x is simpler for simple minds.