Jonathan, I can give you my perspective about some of these publisher's web offerings. I have seen, and have used a few, of these products, in particular, McGraw-Hill, Pearson, and Cengage.
The subtle thing about these publisher products is that they seem to be competing, in a subtle way, with LMSs! As one digs into what they offer, some will provide the ability for you to create content pages, quizzes, and assignments. Doesn't this sound like what Moodle, Blackboard, D2L, and Canvas do? However, it appears that the publishers are not aggressively trying to replace a school's LMS, they are trying to get paid "per student." From what I have seen, a student needs to spend maybe $35 to $50 for the "key," or be forced to buy a new textbook that comes with the key. New textbooks can cost as much as $250 more than used textbooks. So publishers are willing to make their money in a different way.
Some of these products can be configured to seamlessly connect to an LMS, often avoiding a double login. Kind of nice.
But, what can these products do that the LMS cannot do? Well, some LMSs have poor "quiz engines." So, using the publisher's website can be a plus! This is what I would probably do if I had to use my school's Canvas product. But Moodle's quiz engine is great. So I get test banks from the publisher and import them into my course.
Some publisher products provide PowerPoints. But I prefer to create my own PowerPoints, sometimes using what the publisher provides, modifying them, and posting them on my Moodle.
Some publisher products provide online textbooks, sometimes crosslinked to quiz questions. Yep, maybe a nice feature. But I often wonder if this makes it too easy for students to "get the correct answer" instead of "learning." So I am neutral about this feature.
Some publisher products provide a grade book. Well, Moodle's grade book is much more powerful than any of these publisher grade books. However, some LMSs have weak grade books, so the publisher's grade books can compete with these other LMSs.
So, I have not been convinced to go the publisher website route, yet. Simply using Moodle provides me a better educational delivery method. And there is no extra cost to the student for using Moodle. Students can buy the textbook from a variety of places, and if they search a little, at a dramatically lower cost.
Maybe some of what I have said will provide opportunity for others to think, and post. I am always open to other ideas and perspectives.