I don't think they were being totally serious, A. It is just mild conversation.
I wasn't even thinking about AD when I was mentioning forums, I had other sites in mind.
But somehow I do get the feeling if someone really wants to improve their basic grammar, reading and writing in any form is crucial. That even goes for replying on forums when it's a honest attempt at actually writing something. Reading alone (as in being a lurker) would not help much, nor would being a verbose "high" brower.
Absolutely, Naisho. You've got to put your observations into practice. That also brings up the idea of having an audience. Obviously if you don't write anything, you have no audience. Your writing is only as effective and "good" if it is able to reach and influence your intended audience in the ways that you intended.