I get you but I think it's fair to ask. You have no idea how enraging it is to dedicate a whole hour to someone for interview only to ask at the end (you know at this stage) if they used the product and they say "no" and look slightly ashamed and retarded.
Every single place I've ever applied to, I've used their product DEEPLY
But like, you can't ask - you just gotta wade through a million retards.
I definitely do feel that. And I think it's fair to ask/check in, and I agree, there's potentially a big value unlocked when someone is a deeply passionate user of the product.
But when the bulk of your ostensible job ad is making sure you've installed the product, then hmm.