Some will, certainly. The more serious 10N guilds will, I think... say the upper 20, 25% of the current crop of 10N guilds. But the vast majority just want 'some sort of progress' and to raid with people who they know. MANY of those will get that progress in Flex, not normal. Remember, that these are mostly people who aren't invested in being seen as cream of the crop, heroic raiders, by definition. Most of them, or at least the ones I've known, would much rather kill 8 Flex bosses than 3 Normal bosses. Of the Flex guilds, some will also do some normals, but not all.
As for rotation... as I noted earlier I've done this as a RL and officer of a midrange 10N guild in previous xpacs and MoP. Rotating one, maybe two people in is doable, especially if some of them are also tanks and healers. But usually the tanks and healers are close to 100% and it's the DPS that rotates and swapping 8 people in 5 spots when you have 13 people is a pain in the ass. I've been there when the standby person logs off or doesn't show because they figure they won't get in... Pain. In. The. Ass if someone no show. And per boss rotation is just overhead and I hate taking time out of our 6-9 hours a week to swap people around.