I've been seeing a disturbing trend in the photo industry of late: The insistence that every camera that doesn't have full-on HD video -- and now 4K video -- is somehow deficient. In part, I blame this on Canon, for their inclusion of HD video recording capabilities in their 5D mkII full-frame DSLR, effectively creating the HDSLR.
An entire industry has grown up around the HDLSR phenomenon, with enough parts and pieces to build a truly impressive-looking "digital cinema" system.
But, here's the rub. A digital single lens reflex or mirrorless camera is not a video camera. The technical requirements of a still camera an a video camera are very, very different, and trying to combine the two into a single product produces nothing less than a compromised product, incapable of doing both jobs equally well.
While this is not a technical blog, I'll give one example of the shortcomings of DSLR video: Most HDSLRs still suffer from the "rolling shutter" effect, resulting in distortion and an odd bending effect when the video contains certain kinds of motion. This is due to the way that a still-camera scans the CMOS sensor during the exposure, as opposed to the way a video camera scans its CCD sensor (or sensors, in pro-level video cameras). There are many, many examples of the rolling shutter effect on YouTube.
With the advent of 4K video, each frame of video is, essentially, an 8MP image, and some folks got the idea that extracting frames of video for still images would be a good thing -- the ultimate in high frame-rate burst mode -- since you can technically get a 24 or 30 frame-per-second recording from your video, allowing capture of that "decisive moment" and a video of the event as well. But, in reality, compromise steps in to blur the lines between video and stills -- literally. You see, a normal 4K video in a video camera is recorded at either 24 or 30 frames per second, with a shutter speed of 1/24 or 1/30 second. This makes for beautiful, film-like videos. But, these are not action-stopping, decisive moment capturing shutter speeds. To stop that action, you need shutter speeds of at least 1/250 and often much higher. If you shoot video at those shutter speeds, the result are flickering video -- almost stroboscopic. You lose the fluid motion of video.
So, what's the bottom line of my rant? Simply, buy the right tool for the job. If you're serious about photography, buy a camera for its still capabilities. If you're serious about video, buy a proper video camera.