While I shoot more video than stills, I do often fly and shoot stills. Some gopros can do both at the same time, and what you end up with depends on how the camera is set, 5 to 7 megapixel.
It's true the FPV screen does goes momentarily blank during a still shot on the GoPro. At first I didn't like this flying blind for a moment, but you soon realize that's how you know when you took a picture and when you didn't.
You can pull two megapixel stills out of 1080, and four megapixel stills out of 2.7k. I do this all the time. I love the way you can import the videos into Aperture, and then thumb through the video and create a stills from the video, and the stills created inherent the appropriate creation time and meta-data. Creating stills from video without this kind of structure makes it a real nuisance to try and keep them all sorted together and organized.