I finally went ahead and paid the 1000 gold for the dual spec for my Death Knight a couple days ago. So I've had lots of opportunity to check out the new system and I think its going to be everything I had originally hoped it would be. Of course it's something that lends itself to hybrid characters a little more than pure characters, but it also lends itself to those who want a PVE and a separate PVP build as well. For Hybrid characters, you probably really need 4 specs, unlike "parent" characters like Rogues or Mages, who can get by with two (PVE/PVP).
At any rate, I decided I wanted to keep my Unholy spec, which has a couple of non-raiding talents like On a Pale Horse, and Improved Unholy Presence which I use for mobility when travelling and questing. And my second spec is a Blood build. I figured I could run in my Unholy spec on trash pulls because of it's high AOE damage, and then switch over to a much higher single target DPS Blood build for the boss fights. When I'm questing I also travel in my Unholy build.
I will probably pay for dual specs for my two other characters at some point as well. My Shaman is Elemental, and I would like to keep that since he's mostly epic geared. But I had leveled him up as Enhancement until about level 65, so I would probably want an Enhancement spec for him too. And of course there is my Druid who is still level 71. I'll probably dual spec him as Feral/Resto whenever I get around to playing him a little more.
I'm not sure I really agree with Blizzard's imposition of a 1000 gold fee for the dual spec, but all in all, it really doesn't take that long to come up with that kind of gold these days. Other than that, there really isn't any reason not to take advantage of it.