Heh I just dug into this a bunch since I actually noticed the reflection/glare issue at night with my clear visor. But from what I've gathered online, the design of your helmet/visor along with personal preference will dictate whether you want to use it at night or not.
The do not use at night thing is more or less just a liability thing, so forget it.
Just for reference, I have a HJC FG-17. I use a dark smoked visor during the day and put the clear one back on for nighttime. I noticed the first time I went out at night, the glare/reflections were very distracting. I found out later it might've been due to the pinlock, and when I went out again without it on I confirmed that was the issue. It seems like how close the visor is to your eyes along with the angle your visor is at relative to the light sources will determine more than anything how much glare/reflection you see. Then of course it depends how much that bothers you. I personally find it rather bothersome/distracting, so I prefer to ride without the pinlock at night and deal with the fogging.