Unaligned != Any. WotC is king of the Keyword game. Any would mean, like PC races, they are completely flexible from individual to individual. Were as unaligned means they behave as suggested in the "Unaligned" description in the player's handbook. Groups can be "Any" but individuals have to be something.
Alignments in general have little to no purpose in comparison to ages past. As such that means that most of the previous flavor is right out the window. The problem is... WotC has done NOTHING to replace that gap. And the little flavor it has created is disjointed at best.
I agree with K-Dad that if it wasn't broken why fix it? I also agree that if you're going to create the mythical divide between all dragons (Io split into Bahamut and Tiamat) then why not follow that theology through?
By removing the paragraphs of description on creature's nature and behavior and reducing everything down to one or two sentences of description the alignment becomes ever more important to people new to Roleplaying and DnD. Unaligned can be very engrossing and nuanced if given the time, effort and breadth to be so. But when since monster details are pretty much non-existent it's another random thing to fight.
And considering that 99% of the stats presented in the MM are combat oriented it is absurd to think that anyone will not assume a conflict normally leads to a fight.
When I looked at a unicorn, demon, devil, dragon or kobold in the previous editions I didn't just see an enemy to encounter. I saw an actual creature. A member of a fantasy world that could be intrinsically added to the storyline of our game. Now I'm not given that kind of assistance.
When you look at the books as published you cannot discuss those books inherent value (or lack there of) while arguing any house changes. You have to keep the confinements of the arguement to the information as presented. And the information that is presented has be altered in a number of ways that serve no diagetic benefit (or even purpose).
I will also argue that working a Lawful Good antagonist into a story with mostly good party members is a feat of epic proportions that is not only entertaining but unforgettable. When I was a sophmore in high school I played under a DM that forever tainted my trust in Solars, solely because once the story was all said and done I realized that the moral dilemma presented wouldn't have allowed the epitome of Lawful Good to do anything other than what it did. And in turn we had to stop it... for the lesser good.
I agree that the alignment system is antiquated in its own way. In fact, in more character based storytelling it drives me crazy. But it was perfectly integrated into the world of the game. Everyday people knew that there were forces of Law, Chaos, Good and Evil. There were entire planes dedicated to them for (insert diety name)'s sake.
Now they say that their new DnD is pillars of light in the darkness. Show me one. Anyone one printed in a core book. I want to know what it is that the PCs have to live up to. What is that keeps their hopes alive at night, cause a votive to their god isn't going to do it forever. What creature will they see in the wilderness that makes them want to keep fighting for good?