Steve Kaufmann has been having a little anti-grammar diatribe on his blog. So far, there are two blog posts, two podcasts, and a video post. I thought I'd take this opportunity to explain why I fall into the pro-grammar camp. I commented as such on his blog, and a number of people agreed with me. Steve himself was surprised at the number of people taking the pro-grammar stance.
Basically, reading a grammar gives me the details I need when they're non-obvious or sufficiently rare to make extracting them from input impractical. It also allows me to start producing content sooner (i.e, with less input). This is useful for when when I haven't absorbed and internalized the structures because the learning materials don't present the constructs I want to use in sufficient quantities to be able to extract the rules I need. Perhaps its my computer/math background, but I find it useful to be able to abstract away the patterns into rules, and then use those rules to create new patterns. Having the grammar makes that step faster for me.
I find grammar rules like the mnemonic link-words you use when memorizing vocabulary. At first, they're required and the mental lookup takes a while. But after a while, the lookup becomes faster and eventually drops away: you're just left with the rule.
I also find grammar rules to be useful clarifications. In one of the podcasts, Steve quotes some particularly ugly rules from a Portuguese (I think) grammar book. I agree -- statements like that are useful only to linguists actually studying the Portuguese language and not that helpful for someone learning Portuguese. However, from the clarification side, I know I've had a particularly ugly French grammar rule that mentioned that this other case I was having trouble with was included in this rule. There we go: I already knew the rule, and the grammar let me collapse these two cases that were actually the same back into a single mental rule.
A grammar, like a phrase book, puts me on the path to independent production sooner than if I had input alone. It also lets me read more effectively because instead of just glossing over sentences and getting the gist of them, I can actually analyze them for what they're saying and notice the rules I've learned being put into practice. Unless I'm reading carefully like this, I'll just end up skimming and saying "yeah, I kind of understand what they're saying because I know this word here, and this word over hear looks a bit like this, and I think this might be the past tense and here's an auxiliary verb at the start." Knowing the grammar lets me view these kinds of sentences as clearer examples of what I've just learned, and then I can put that knowledge into practice sooner.