I feel compelled to respond since I am on both sides of the hotel business.. I own a tiny hotel in the Luberon and I also fly about 50k miles per year (staying in hotels along the way.)
Booking.com has been amazing for us. They take 15% but it is completely worth it. We don't have "a name" nor do I have time to invest in the relentless SEO content marketing necessary to differentiate us when people search "hotels in the Luberon" or "Provence hotels." Since we are a very small place (just 3 suites,) it's an unprofitable use of my time to be generating blog content and engage in full-court-press inbound techniques -- so unless you are a chain or happen to be featured by travel writers, good luck being discovered via inbound searches.
We do have a website and a good direct booking system, but we probably get 3 bookings per year directly from our website yet between Booking.com and cross-listing on AirBnB, we are completely booked through most of the season. Our booking software is integrated with Booking.com and AirBnB so we are able to have all of our channels coordinated with a minimum of work. If one considers Booking.com as a lead-generation system, 15% is a huge value. I would easily spend far more if I were marketing directly through ads. Our TripAdvisor page is free -- we haven't paid the yearly €799 fee for an enhanced listing but TripAdvisor still is great for us since we have 4.5 stars.
Guests from Booking.com are far less work than from AirBnB or direct. AirBnB customers seem to still not understand what Instant Booking means -- we get almost daily inquiries of "do you have availability for some date" -- yes, obviously that's why you are able to instant book! AirBnB guests tend to want to have long conversations while Booking.com people just book and show up with a minimum of fuss. We also have a very liberal cancellation policy, unlike many places in the region -- we found that just letting people cancel whenever they want works out fine in the long term because are area is high demand so cancellations don't really cost us anything because we quickly get rebooked during peak season. So everyone is happy. In terms of taxes -- our prices have all of that included.
My point is that sites like Booking.com and AirBnB have made our business -- we essentially incur no marketing costs. So we love Booking.com! It honestly doesn't matter one bit of you book directly with us or via Booking/AirBnB -- our prices take our costs into account so you aren't "hurting" us if you book through a third party. We are just glad to welcome any guests however they find us. The big chains probably get hurt the most by third party booking sites because they are already spending money on marketing and have name recognition, where for ultra-small-timers like us, you probably would never have heard of us without those third party sites.
The short version: third party sites add huge value to our business -- they get to do the hard work of marketing us while we can focus on the guest experience.