Maybe you'll use your roof more than a lot of people do. But I would say 9 out of 10 folks that I've worked with that installed roof racks for seating don't end up using them for that. Being 8 plus feet off the ground is not particularly pleasant for most folks.
And as far as stargazing goes, if you're in a spot where you need to be 8 ft off the ground to see the sky, I don't think you're working hard enough with your vans mobility.
Of course if you just like the way it looks by all means go for it.
It's mostly a quirk of mine, I I'm a bit irked by folks who put enormous amounts of effort into justifying what was simply aesthetic decision. For example five or $6,000 of lights on the front of a campervan. "You got to have them for safe night driving!"
As far as storing watercraft, I think there are a couple better designs than a simple roof platform. Both for cost, weight, and ease of use.