I don’t have any research, but I think you can test much of it yourself. For example, last season I was on a MV plan, i. e. Mondays and Fridays were rest days. I had a group ride buddy for a light Friday workout (endurance workouts like Carter). I just couldn’t sustain that.
This season I created a MV+ plan, which extended all workouts by adding Z2 workouts and upping the difficulty levels. This seems much easier than adding a simple Z2 workouts on Fridays. I really look forward to my two days off, mentally. I can sleep in, I can just relax, knowing I have done my work for the day. The next day I am ready for the next workout with plenty of enthusiasm to spare.
Now I still commute every day by bike, and I do that in Z1, Z2 max. But it is only 4 km one-way, so it doesn’t add much effort.
Coming back to your buddy, it all depends what “light” really means. If that means 30 minutes at Z1, I guess that’s not much of a problem. But personally, I don’t think that sounds sustainable — especially once you think Z1 spins are mandatory.