If you've got the numbers to show that this sort of thing leads to more active and involved users then I can't really argue against it but you should stop pretending that this is the only way you could possibly give a good first experience. Off the top of my head it seems you could have an awesome first experience by starting with a bare minimum style quiz. I haven't seen what you have so far but odds are you wouldn't need the entire quiz you currently use up front. From that quiz you can present a page with a handful of selections that would fit the user's responses, giving them a rough view of what they can expect. From there you can prompt the user to properly sign up and take the full signup.

I have no idea what the numbers would look like, but it would certainly provide a pleasing no-registration initial experience.