When we went to pick up our van, a vent had been installed. It was circular and located at the front of the popup, behind the driver, extending a couple inches higher than the top of the van.
I asked them to take it out, which they did, reglassing and repainting.
I wasn't as worried about the leaks, given the lifetime pop up warranty against leaks, but I couldn't take the height since I was scraping to fit the van into my garage as it was.
If I lived in a hot area or did much desert camping, I think I'd find a way to add a vent. It seems like it could be a cost effective way to get more air circulating.
My 2 cents.
2008 EB-52, Quadvan 4x4, 6.0 PSD SOLD
2017 Ford Transit 148WB mid-roof