While I agree with you that Free Trials may be beneficial, I think what's tainted in people's mind is how usually is to try to convince you that the game is worth it when he is not anymore (meaning it is a little late for that). Which gives them the perception of the "Games only do that when it's bleeding". But that's just my opinion into other people's mind, specially in the MMO market.
Now GW1 has a free trial alright but when I got it (probably around early 2009) I'm not sure I saw a trial at the time. Took long enough for a non-sub game to get it rolling right?
Now yes, Anet will probably do it but I would guess way later. And at some point it will be beneficial, but on a game where the primary reason for the hype is the actual gameplay and how much fun you get from it, considering we are talking about a MMO with no sub fee. I think it's only "natural" that players will take up the risk, I mean I'm pretty damn sure people took bigger risks in gaming history for much less.
And as far as single player demo's go. A established company will not release a demo. Now a new company or a more.. exotic title might give out a demo depending of the strategy but most companies (assuming they fit one of those 2 points I said) WILL wait at least 3 months to give a demo because that's when they make most money from sales. Taking a look at Anet (and NCsoft) and specially the huge success that Gw1 was they do not meet those points. So I think it would take a fair amount of time to get a demo going, with so many hype, marketing strategies and much more to come I think they have long ways away to have a major benefit from a demo, specially with the "involuntary" media out there like gameplay steams and stuff, which might not be the same as playing the game but hell if you don't feel itchy about getting to it. (Hell I own the game and I feel it).
Thats what I think tho