But only 25m casual groups or 10m groups with large rosters, right? A small, casual 10m group that ends up missing people on a given night won't really be able to take advantage of the flex system to "just raid anyway" with their friends, which I think is a shame.
I'd love to see a little more flex allowing for maybe 8-28 raiders or something, so that 25m groups can also take advantage of the new flex raids to not have to bench friends, and 10m groups can worry less about having their raid night cancelled because of one or two missing raiders.
Raid mechanics start to break down when you go much below 10 players. There are still assumptions of having 2 tanks and 2 healers, and some mechanics are not really able to scale with much fewer DPS. Raids are still going to be raids, and raids are designed with some 'givens' to ensure the mechanics can be interesting, complex, and challenging.
But unlike Normal and Heroic, you CAN
invite BattleTag/RealID friends to a Flex raid of the current tier, and so if you're short you can fill it out with people you know, instead of spamming chat to pick up randoms.