On the other side, I've never heard of an interview where the candidate is given real work. Sure, they might ask about a problem the company has faced in the past, or is facing, but it's unlikely the response is used in any meaningful way, other than determining the candidates expertise and methodology for approach that type of problem.
How many hours of research, experience and interview prep is the candidate bringing to the table? When you can answer that, you can make a determination on whether the company is dedicating more or less than the potential hire.
That being said, for non FAANG style interviews, prep shouldn't significant. That being said, I'm agreeable to accepting both parties have a sunk cost and no interest in wasting their own, or the other party's time.
One of the reasons it's staggered is so the company or the candidate can end it early if it's not a good fit. Less time wasted this way.
Frankly, I'm kind of shocked by the resistance HN has to spending a couple hours interviewing with a company. You're going to spend several years of your life working with this company. Is it really a dealbreaker if they ask their candidates (included your future coworkers) to participate in a few extra hours of discussion and confirmation?
In the real world, I offer to schedule interviews on lunch breaks, before or after work, or on weekends if that works better for people. Practically speaking, most developers (especially WFH/remote) have zero problems finding time during the day for a 1-2 interview session. That's equivalent a long lunch break or a quick errand.
In practice, I've never had anyone decline an interview for lack of time and very few people ever take me up on my offer for flexible interview hours of their choosing. Most people are eager to get to talk to the company, hear about the job, and show the company what they can do.
It is crazy to me to casually drop that it's standard for a company to get a free day from someone they're interviewing and may just never contact again.