I've had several cases of people looking up my email address from my website and emailing me with a question: it amounts to trying to queue-jump - and of course if you answer it you won't get any points (assuming that you might in the forum, which is a long assumption) and, perhaps more to the point, other people won't see and possibly benefit from the answer. I generally answer once, but make it politely clear that I'm not prepared to run an email clinic. Only very occasionally, usually when there's been a privacy issue, have I invited someone to email me.
Private Messaging is less intrusive because it doesn't pop up at you when you're not on the forums, but with a very few exceptions I think it goes against the spirit of co-operation which should inform a forum (no pun intended, for once).