But the iPhone 6 Plus does and it has even higher remarks for its camera, same with the Note 4 which also has OIS. To me, a flagship smartphone in this day and age should have OIS. Good images can be taken with smartphone cameras lacking OIS, the standard iPhone 6 is a good example of that. Even the S5 is a good example of a camera that can take nice shots and it doesn't have OIS. Having said that, HTC should have used a camera with OIS in their M9. The M8 had a cutting edge ultra pixel camera. Although the resolution count was too low (4MP), it was a step above and beyond that of regular smartphone cameras at the time. Why did they do the same thing with the M9? Instead they did what many manufacturers do: mindlessly up the MP count and call it a day. Adding OIS would have greatly added to the camera's functionality in the M9. Instead it's just a regular ol' smartphone camera now no different that what we have been using the past few years.
OIS also helps when capturing fast moving objects like dogs and children. My fiance's Note 4 has OIS and after using it for a few months, I don't think I could go back to a camera without OIS. I've tried with my S5 but her Note 4 takes clear shots of our dogs while mine doesn't, her Note 4 takes clear shots at night while mine takes grainy shots with overblown colors. I know the Note 4 uses a better image sensor but OIS also helps make those shots better. I even saw an improvement with my Samsung compact mirrorless camera going from a standard lens to one with OIS.
Yeah, it may not be a required feature to get good shots but it does help and really, at this point, there's no reason why HTC shouldn't have included it.