This is what is frustrating for the people who have been with this series for a long time. We know how much blood, sweat and tears GRRM has put into these books and into ADWD in particular, then people dismiss that in a second because he's taken the same amount of time, or indeed less, to write ADWD than he has the other books in the series. It doesn't really make any sense. If he took over four years apiece to write A Game of Thrones and A Feast for Crows and over three to write A Clash of Kings, why is there all this despair and gloom over him taking under four (so far) to write A Dance with Dragons? I call this 'consistency'. People keep using the short writing time of A Storm of Swords as the rule, not the exception, which I really don't understand.
In fairness, the above information is not massively well-known. People just see AGoT 96, ACoK 98, ASoS 00, AFFC 05 and ADWD 09 or 10 and start wailing. I can understand that, but it's not really warranted. At least not yet. If ADWD still isn't out another two years down the line, perhaps the hysteria will be more understandable. A tad disturbing ("It's only a book!"), but understandable
I think that's a natural feeling and I think it doesn't help that GRRM's POV on the issue is different from that of many newer fans, that is the people who've only read the books in the last year or two and immediately gone rushing off to his blog to find out the latest on the next book only to find no mention of it in a year and a half except for a single entry back in February and lots of talk of other things that they are not particularly interested in. GRRM's POV, on the other hand, is that he has talked about and discussed in-depth the series non-stop for over ten years, provided (comparatively) large amounts of information during the writing of the previous books and it hasn't stopped people moaning to him about the situation, and he's not really prepared to carry on doing that any more. He has other irons in the fire that he wants to talk about as well, and if people don't want to read about that then they don't have to.
I also think GRRM's attitude is one of bemusement at being a fan of a particular work and not the author. When I find an book I really like, I check out what else the author has done, and largely find that I like that as well. After reading ASoS I went out and tracked down Fevre Dream and enjoyed it as much as ASoIaF. Whenever I therefore encounter someone who says, "I love ASoIaF but I'm not reading some vampire book!" I am somewhat confused (especially since GRRM has undead in ASoIaF as well). That said, I don't think GRRM is naive enough to expect everyone who likes his big fantasy series to like his non-big-fantasy projects as well: he's talked about Stephen Donaldson's fortunes after he stopped writing Big Fantasy books in favour of SF and his increase in sales after he returned to fantasy.
In all of this it should be remembered that, by themselves, the success of the Wild Cards books and his pre-ASoIaF novels would make GRRM a well-known SF&F name with a solid career and able to make a living from his writing. Therefore there are a lot of people out there interested in the other projects. Not as many as the ASoIaF fans, no, but enough to sustain a career many authors would be envious off.
This is a key point. GRRM never said that ADWD was 'nearly finished' when AFFC was. He said he'd chopped material out of AFFC to go into ADWD and ADWD was thus half-finished. He still had to write the other half. In fact, the choice of the word 'half' wasn't a great idea either, as the numbers he gave for completed pages when AFFC was finished suggested the transplanted material was actually one-third of ADWD, so he had even more all-new material to write from scratch. Then ADWD turned out to be longer than planned, so at the moment it may be closer to a quarter. And then he seems to have rewritten most of that material.
So yes, GRRM definitely made a few over-optimistic PR errors when he finished AFFC, out of the jubilation of finally finishing that novel and getting it on the shelf, but events were overtaking him even as he was making those statements to ensure that ADWD wasn't going to be out as soon as he'd hoped.
I was relatively new to the series and never really read his blog for information because I was actually going by a tentative date on wikipedia and stalling my reading of AFFC. When I finally finished it a while back, I gave his blog a look through and read the Feb 19th entry and that was enough for me to be patient and wait it out. I used to be under the impression that all novels took 3 or 4 or more years to complete just because books I had read before had wide publishing gaps. Its like you say though, too many people dismiss all the reasons he could have behind the delays and just cling to a quick release of one book as a norm and expect it to continue.
The only reason I would have in thinking the series might not be completed is the time gaps might get larger as the "things on his plate" multiply. That combined with me having a history of bad luck just has me thinking "because I'm interested in it, bad things can happen". Like a 'world revolves around me' type of outlook on life but not in a selfish way, more in a pessimistic way lol. That said, there are the exceptions. Shortly after I started reading the series I read HBO was going to produce a series. Just goes to show that any expectation you have in life you can throw out the window.
All we can do is sit back and wait and see what happens. Maybe send the man some support mail because lord knows he needs it with all the impatient fans about.