TL;DR ALERT
Maybe you should educate yourself on megapixels and what HTC is doing with the dual camera.
1. Megapixels means that you can crop into an image, and the camera captures more details. Increasing Megapixels is only useless when the sensor characteristics do not change to match. The S4's Sensor was better than the S3's. It took in more light. It was faster. The CPU is stronger. Samsung used a custom ISP from Fujitsu in the S4/Note 3 camera systems. It is not as if they simply took the S3's camera and bumped it up to 8MP without any other modifications.
2. Ultrapixels: Practically Useless in Good-Moderate lighting (output from flagships with smaller pixels, like the iPhones, GS4/5, Note 3, G2, etc. are proof of this). A lot of the benefits of the UltraPixels are completely destroyed by the image resolution since you are getting a brighter image, but the detail is left wanting - VERY wanting. If those were 8MP cameras, they would have solved the issue HTC was trying to Solve, which was offering detailed images with less noise and more light in darker environments. All they did with the One was offer a brighter image, but there is still a fair bit of noise and they gave up a TON of image detail with the resolution. The Pixel sizes are great. The image resolution was a massive fail. With those other phones you can take a darker picture, adjust it in an Image Editing program, even resize it to mask noise and print it. In many cases, the output will be superior to what you get out of the One due to the greater detail in the images. Additionally, their processing algorithms are quite aggressive, and they have a history of compressing images more aggressively than other OEMs do (which is why Samsung phones tended to throw out noticeably larger photos than HTC phones of same camera resolution - upwards of 1MB larger... in size, even).
3. 4K Video: One Frame of a 4K video is ~8MP. They are more detailed than the full images out of HTC's camera and the Note 3 does 4K quite well. Why should I deal with camera settings and trying to get the picture exactly right at that moment, when I can simply shoot 4 seconds of 4K video and Crop a more detailed image out of the video that I'd get snapping with the One's camera. This works 95% of the time, unless you're unlucky and the specific frame you want is a blur (which is only a huge factor for something like Sports, but for that you'd likely shoot 60 FPS 1080p (Smooth HD) anyways).
4K is the future. We like to be future proof. Also, 4K video processed (downsized in editing) to 1080p blows away any 1080p native video from a smartphone. It isn't even a contest.
4. Depth of Field Adjustments: Samsung has already displayed this on their Single ISOCELL Camera System, which is more innovative than HTC's UltraPixel system (which is nothing more than a Low Resolution Sensor with Low Image Resolution AFAICT). HTC does have OIS which matters more to some people than others, but is still an advantage over Samsung in their camera system.
5. There is some 3D stuff they say they can do with the second camera. Not sure how useful that would be on a day to day basis (real-world usage & benefit).
At this point in the progression of Imaging and Video on smartphones, the need for "UltraPixels" is so niche and situational that it's hardly a selling point. I cannot for the life of me figure out why HTC went from the HTC One X which took decent low light image and fantastically detailed image in normal to great lighting to this UltraPixel camera system, which spits out images that pale in comparison to other smartphones using higher resolution cameras. In the day and age where people can order prints from Walgreens, Wal-Mart, or CVS right from their smartphone. In a day where Smartphones are taking video that allows you to crop images on par with 8MP cameras from the video. It just doesn't make any sense.
I also have issues with HTC using 16:9 aspect ratio in their camera, since not all images look great in that ratio and if you need 4:3 it means the image resolution suffers further.
That being said, people who want higher resolution cameras are not who HTC is targeting with these phones. They've stated that they think most people view images on the phone and on smaller screens, and at those resolutions their image output looks "fine," which is actually true. The images look fine until you put it up on a 22" monitor and start cropping parts out of them, or printing them, in which case they aren't even competitive with most other flagships on the market these days. Those images look like they come from a Mid-Range phone at 1:1 size.