I also experienced another aspect of Flex that I hadn't considered until this week. With my paladin being in a "normal mode" type raid group (hunter in a heroic group), we're at 9/14 N. But with part 4 of Flex out yesterday, we took a night to go do part 3 and 4 as a raid team of 12-13 people and get some experience on those fights. Certainly we didn't play perfectly but the encounters were forgiving enough to allow us to down most of the bosses in part 3 in 2 or 3 shots. Then in part 4 we had time to do Siegecrafter and Paragons, but ran out of time for Garrosh. The great thing was, while we're not yet at Siegecrafter in normal (although I guess we could if we diverted there instead of working on Spoils), we got some experience for people running the gauntlet on Siegecrafter and figuring out who could do it and if they could solo one of the weapons, etc, etc. It was valuable experience that I'm sure is going to help us down that encounter quicker on normal mode.
Likewise, Paragons is a fight with a thousand different mechanics so getting people used to those on Flex before they get there on normal was valuable. We ended up one shotting it fairly sloppily burning all our brezs but still people got to see it all. We'll probably go back tonight and go through Garrosh on Flex to give people a shot at their heirlooms if Garrosh is also fairly forgiving on Flex.
So Flex is now also a nice training tool for raid groups working through normal mode.