I have no doubt that this will be a controversial statement. Especially among the hard core Android fan base TouchWiz and Sense are considered heresy. Some of the criticism that these custom OEM skins receive is left over from the days when early versions of MotoBlur, TouchWiz and Sense were slow, clunky and hideous. Some of the criticism that custom Android skins still receive is totally valid. Sometimes TouchWiz does have a bit of a lag and Sense has menus that are poorly constructed that can make reaching basic features be difficult. But before you go flashing AOSP to your new Galaxy S4 or your HTC One there are a few things you need to consider.
The version of Android that comes on your new flagship smartphone doesn't just have a different look and feel than stock Android. The manufacturer has taken the time to tweak Android to run that device's specific hardware. Especially when it comes to features like the camera, flashing stock Android may do your experience more harm than good. For instance, if you flash AOSP to your Note II you will find a drastic drop in the quality of any pictures you take. The same is likely true for the HTC One. HTC specifically designed the software on the One to work perfectly with a unique technology and lens. If you switch out that software with the generic Android kernel there are going to be consequences. Maybe you don't really take that many pictures and having a stock Android experience is worth the sacrifice. In that case, by all means do as you wish. That is the great thing about Android, you can make your own decisions.
We also need to look at this issue from the perspective of a company like HTC or Samsung. Lets face it, there are a lot of Android devices out there and although well-informed fans (like you) may compare benchmarks and specs meticulously before making a buying decision, most of the public (aka, a market worth billions) doesn't really care. What the public wants is a cool looking device that can do cool things in a cool way. They want cool phones that are cool. If someone sees a feature like Smart Pause on the Galaxy S4, thinks it looks neat, buys the device and then immediately disables Smart Pause guess what just happened? Samsung just sold its 3 millionth Galaxy S4 that day and that customer got a fantastic smartphone. TouchWiz may look a tad cartoony for my taste, and the slight lag may get on my nerves every once in a while, but that doesn't mean that most of Samsung's customers won't love the experience, the look and the responsiveness of TouchWiz. A company like Samsung spends millions every year figuring out how their customers make buying decisions. If no one wanted TouchWiz, Samsung would get rid of it in a heartbeat.
I hope that this has given some of you a new perspective on OEM skins. Keep in mind that these customizations are exactly what Android was created to support, and adding value to a device through software features isn't a trend that will go away anytime soon. The next time you unbox a shiny new Android smartphone give the manufacturer's software a chance before you start flashing away.