I absolutely agree. I think that it's important to first establish interest and weed out candidates that are obviously unqualified. You can figure a lot of that out in a short amount of time. While some of your best candidates may already be employed and thus only casually interested initially, at least for someone like myself that interest could increase significantly after sitting down and chatting about the company and projects for a bit. If I were to then become really interested it could be worth spending a lot more time in an interview process.

Personally I don't think that a company will learn much more from 8 hours or more than they could learn in 3-4 hours tops though. I think it's overkill in most situations.