Hammerfast is basically a backdrop for players to use as a home base as they go off on other adventures. Most of the details are focused on describing the city itself, with a paragraph or so on major businesses, and a lot of color detail that keeps the city interesting enough that it's not just an inn and a weapon shop to the players. There are a couple villains and suggested adventures based around the city, including a pretty nasty high-level dragon, but not really enough to make anything but a barebones story for a multi-level campaign. For what it is, Hammerfast is a very nice product, and I recommend it highly.
Now I feel like you might be misrepresenting exactly what you get with Hammerfast...
As far as antagonists in Hammerfast go... here's just a few and it seems significantly more than a handful, and quite a few based in the actual town of Hammerfast as opposed to just the surrounding area...
1. Serlek's Gang
2. Carthain's Gang
3. The Rockguard Clan
4. Joren Deathkeeper
5. The Circle of Stone
6. Thar
7. Calastryx
8. Trade Guild
9. Lore Guild
10. Craft Guild
11. Orc Tribes: Bloodspear Orcs/Weeping Skull Tribe
12. Queen Stravalla of Winter's Mourning
13. Spider Goblins
14. Swiftriver Halfling Clan
15. The Deep Burrow Gang (kobolds)
16. Barrthak the dwarf lich
17. Baldreg Skullbreaker the high priest of Gruumsh
There are also many that I didn't list that can become anatagonists or allies depending on the actions of the PC's. Just reading through the book there are tons of adventure hooks in the actual city... from the political maneuverings of the Guilds, the goals of various NPC's, the machinations of the Stone Circle and so on. So I disagree that Hammerfast is suppose to serve mainly as a backdrop and homebase for adventuring in the wilderness.
Neverwinter is not a backdrop--it's a campaign construction manual. It has half a dozen major factions with distinct themes and leaders, and detailed information on the leaders plots. It also has a lot of space devoted to how those factions interact with the players, and with each other. It also has has at least half a dozen smaller factions who play supporting roles to the players and their enemies.
So again... it's an argument of more, not necessarily anything they do differently, since the goals and plots of the antagonists I listed above are all in the actual Hammerfast product. Now Hammerfast does (and IMO rightly so) leave how these factions interact with each other and with the players up to the DM... but that's a question of what do you want decided for you and what do you want more freedom with.
The city itself gets quite a bit of page space, but focuses on bigger-picture locations that are likely to feature as adventure spots and player destinations (Castle Neverwinter, the Graveyard, the spellplagued chasm and the Wall that surrounds it)--unlike Hammerfast, it doesn't really linger on color details about the local seamstress and alchemist. The book also details half a dozen other major locations, and numerous minor ones, that surround Neverwinter--and explains why they are important to Neverwinter and to the players.
What do color details have to do with whether something is a campaign setting or a backdrop? IMO, Everwatch in Dragon magazine is a backdrop and homebase for PC's... neither NWCS or Hammerfast are this.
More and more this seems like a taste issue. I find those details you dismiss are what makes the setting of Hammerfast come alive for me... while telling me what the interactions and relationships of factions is something I want the ability to do for myself in a sandbox...YMMV of course.
Oh, and for the record... Hammerfast also details locations of adventure around it such as Two Roads, or HighPeak among others. So the difference again seems to boil down to more.
All of this is linked together by the Character Themes that are provided to players. The book is littered with sidebars explaining how to tie character Themes to the locations and people of Neverwinter--in other words, how to build a campaign. And the Theme descriptions themselves give the player that chooses them a strong concept of their character's background and motivations in the campaign.
Now this I can somewhat understand because there are no themes in Hammerfast, though honestly I find themes a good addition to 4e... I also find the ones in NWCS slightly limiting since there are only so many themes and they are very specific... if you don't particularly care for any of them or they don't fit your character concept then they don't matter. Moreso, IMO, since Hammerfast is set in the default world it's easy enough for the players and DM to tie almost any theme into the setting and this provides alot more variety... and allows for as tight or loose a connection as the DM and players want in your campaign.
So when people say things like "Neverwinter doesn't tell me how to build a campaign," or "It's just Hammerfast with more locations" without mentioning the Themes, that tells me that the people talking haven't actually read the book. Hammerfast is a location--Neverwinter is a campaign. It's much more useful for a DM and their players build a tightly focused campaign than any previous campaign guide. If you just want a colorful location to base your adventures that you're creating or getting from another source, Hammerfast is a great product. If you have no interest in actually running a Neverwinter campaign, you probably shouldn't buy the Neverwinter Campaign Book. But if you want a book that tells you how to put together a Neverwinter campaign--one that can be used over and over and produce a different campaign every time--then Neverwinter is a great resource, I think.
So it seems your argument rests mainly on the inclusion of very specific themes? Fair enough, though for $10 I can get more than enough themes and backgrounds to tie vastly more character concepts into Hammerfast along with a multitude of other stuff... vs. the cost of the NWCS. So again, IMO... Hammerfast was a better bang for my buck.