In my mind, there’s a difference between grammar-only methods (like most classroom settings I’ve encountered), grammar-light methods, and no-grammar methods.
In many classrooms there’s such an exclusive focus on memorizing grammar rules that students don’t get enough exposure to the actual language to internalize it. It’s clear that at some point, the language must somehow be internalized, because we don’t speak our native language through a series of calculations, as grammar is taught.
Instead, I feel that grammar is more efficiently used as a reminder or a hint of what exists, and what patterns may occur as you read. This way, it becomes a helpful pointer toward what patterns you might notice. Sometimes it can be hard to notice those patterns on your own, if they have too many exceptions. Once you’ve had it pointed out though, then it becomes much easier to internalize it through extensive reading, which is where Krashen’s advice becomes valuable.
That said, if the patterns are already familiar, then you can just read. I’ve never in my life looked at any Dutch grammar, I just read parallel texts to learn it. The ideas were similar enough to German that I picked up the patterns very easily. And I was able to learn to understand German quite well without reading about grammar as well, although I did remember the basic conjugations from my high school classes more than a decade before.
In the coming months, as I start to get acquainted with Polish, I’m going to follow a similar strategy, even though the grammar is apparently much more complicated and unfamiliar. I’ll look at some simple examples just to see what’s out there, and then as I read I’ll mentally make a note of the patterns as I see them, and that way I’ll become familiar with them through exposure. I don’t think there’s really a need to suffer through a bunch of boring textbook exercises. I plan to just keep doing something fun.