Matt:
I suspected that was the reason as I stated, although I also wonder whether or not some people that might have logged on more often (hoping?) to see a PM, otherwise won't.
Here's how I think it might play out for that type of person. They aren't on IHub all that often, but when they do, they might make a post or two. I suspect everyone wants to be acknowledged for their thought(s). So, they might log on two or three or four more times over the next couple days to see if anyone responded to the post(s). On the other hand, if they don't get an email, they don't log on at all.
That's how I think it is more likely to play out, but I could be wrong. One thing for sure, I have no way of identifying with that person since I'm an addict!
Also, I suspect that, as you intimated, this is merely step one in the big picture of email marketing. Initially, (pause... should I be saying this out loud?)... you get them used to you spoon feeding PM's to them. Secondly, can marketing anarchy be far behind?? Assuming that to be the case, if I were you, I would definitely do as you indicated you might - that being making the default to "on". If you are going to parlay it into other things, you have to get as many people as possible used to receiving emails from IH/Advfn IMO. If you leave it "off", I doubt very many people will opt for it and it might take a long time to get educated as to what the benefits of that type of communication are to IHub.
Anyway, good luck. Keep the improvements coming!
Len