You have kinda answered yourself in that indeed opinions are mixed.
A cover will protect the paintwork from the ravages of sun , rain, leaves and anything else winter can throw at you and if it is a breathable type, then there should be no problem. It also has the advantage you can leave the blinds up and not worry about security. It also stops pools of water forming on the roof and trying to find any crack in your mastic.
However, some people feel they can scratch the windows and as they are breathable, dust can penetrate the fabric and sit on the roof. They also feel that even if the van gets covered in slime and tree debris, this can be easily washed off in Spring.
My own experience is that I have always used a cover and am happy with it. However if your van has windows that are sloping, for example at the front, then cover these with a soft sheet before putting on the cover. I have found no problems with vertical windows. It is also quite pleasing to pull it off in Spring and a shiny clean van appears. I also looked at the topside of the cover after a winter and it was covered with bird c..p, green mould and all sorts of garbage, the van wasn't.
Basically, at the end of the day the choice is yours