I don't like this advice. For one because I would have missed out on some incredible books. (the first one that comes to mind for me is Roberto Bolaño's 2666, I struggled with the first part, the rest blew me away).
But more importantly to me this seems like a mindset that centers around quick gratification. Perseverance is a useful skill. Thinking the author needs to get you in the first 50 pages is somewhat entitled attitude. The restaurant metaphor seems apt because it seems like taking the "the customer is king" attitude to reading. There's humility involved in reading that requires to give an author time.
I think this is exactly why people have a hard time with this advice. We're used to thinking we owe the author something because we've chosen to read their book. What this presupposes is that we owe them nothing, in fact it's up to them to justify spending our limited time and attention on their book, rather than in the million other options for books available to us.
You will occasionally miss out on a good one by following this advice, but in the end you'll read so many more good books, because you'll spend less time halfheartedly slogging through books you hate and dread.
I think that "first 50 pages" just gives you a poor sample. I often read 5 sections of 5-10 pages from several random parts of a book, including closer to the final.
For fiction in can give you "spoilers", but I don't care. A book which is worth reading to the end is usually worth re-reading anyway, despite your knowing the entire plot. "Hamlet" can be seen as a detective story, but we don't read it to find out who is the murderer.
A book that can be destroyed by a spoiler is not worth reading, to my mind.
In non-fiction spoilers don't exist. But looking at the farther chapters you can see if the author is going to tackle some new and interesting material. First 50 pages of many non-fiction books are often intros, and if the topic is not new to you, you already know what they are trying to explain, and can skim / skip.them.
For me it comes down to trust: if I trust that the author will deliver on a good book (by recommendations, by its reputation in a community, etc.) I will not give up so easily on it. But if I have no reason to trust, why should I waste my time?
I agree with you that we should generally give authors a chance and I would also have missed out on some great books if I would not have kept going. But I am also a very slow reader and therefore have to be very selective. So I rely a lot on opinions from reviewers and other readers to give me a hint that the text might grow on me, otherwise I cannot possibly give any book the chance it may deserve; there are just too many other great books that I would never get to read at all.
That's great. Especially if you have a lot of people telling you this is an awesome book worth reading. I've definitely put effort into reading hard books because of this. But mean, 95% of the stuff I read isn't this. It's still useful to glean some info out of but there are only so many masterpieces in the world.
But more importantly to me this seems like a mindset that centers around quick gratification. Perseverance is a useful skill. Thinking the author needs to get you in the first 50 pages is somewhat entitled attitude. The restaurant metaphor seems apt because it seems like taking the "the customer is king" attitude to reading. There's humility involved in reading that requires to give an author time.