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Thank you. I haven't read the book, which prevents me from disagreeing with its conclusions without being an ass.
I'm going to disagree now, but I still haven't read the book. I'm relying on your interpretation. So perhaps that's worth half.
My half-assed opinion: /m is usually irrelevant to what people expect. The vast majority of programs end up reading things line-by-line. If you're only looking at a line at a time, then /m has no effect.
Therefore, it is only relevant in the less-common circumstances when you have embedded newlines. I find that in those cases, I sometimes mean one thing, sometimes the other. As a result, I find /m sometimes useful, and sometimes it does the opposite of what I want. So I use it when it is what I need, and when reading such code, the existence of that option is a very useful indicator of what the expression is doing. Using it all the time, even when it has no effect, harms readability.
As for /s, consider this common idiom:
What does that do? Well, it looks at a config file for a line saying "Title: The Adventures of the Were-Gerbil" and pulls out the title. If that included the newline, I would be rather annoyed. It's like asking someone "what's in the bag?" and receiving the answer "an apple, a sandwich, and a bunch of air". Smartass.
Also, similarly to /m, I use /s specifically in situations where I want to do something unexpected with respect to newlines. I prefer using it sparingly, so that those situations are more noticeable. Most of the time, I'd rather use something like [\w\W] rather than a trailing /s, especially when what I really want is a mixture of both possible meanings of '.'.
Always using /x, on the other hand, makes sense to me, even though I haven't switched over to it yet. Even in the absence of comments, I often like using it simply for readability: /\s*\((.*?)\)/ is just a lot harder to follow than /\s* \( (.*?) \)/x, and the latter could get away without a comment, IMH(-A)O.