At BlizzCon, we'd discussed some improvements to our Flexible scaling system that would minimize the existence and impact of any real or perceived "breakpoints." Our design goal has always been for the system to be neutral with respect to group size. Specifically, you should never feel encouraged to turn away interested players whose skill/gear level matches the rest of the group's, basic role composition constraints notwithstanding. But at the same time, you shouldn't feel like you need to go out and grab warm bodies if you have a perfectly serviceable raid already assembled. Clearly we have not yet fully realized those goals.
In general, the existing healing/damage scaling slightly favors larger groups, since boss health and damage scale up more slowly than does raid throughput. To use a specific example, an 18-player raid with 12 damage-dealers will kill a boss slightly faster than a 10-player raid with 6 damage-dealers (assuming equal skill/gear between the two groups). But it is true that currently some specific abilities exhibit breakpoints, where adding an extra player causes an extra add to spawn, or an ability to strike an additional target.
To help smooth out difficulty scaling as you add or remove players from your Flexible raid, we will be implementing one of our planned Warlords changes to the scaling system ahead of schedule. In the near future, several boss abilities that target more players as the size of a Flex raid increases will use weighted randomization rather than strict breakpoints.
An example to illustrate what I mean:
Today, Sun Tenderheart's Shadow Word: Bane ability afflicts 2 targets if your raid size is between 10 and 14, but begins to hit a third target at 15.
Once our upcoming change is in place, if you have a 13-player raid, there will be a 60% chance for each cast of the ability to hit 3 targets, and a 40% chance for it to hit 2. If you add an extra player, and thus have 14, there will now be an 80% chance for 3 debuffs to go out, and a 20% chance for only 2.
In short, with respect to this particular ability, adding an extra player will always give you an average of 0.2 extra Bane debuffs. No more breakpoints!
There will be a handful of exceptions where breakpoints remain necessary. Imprison on Sha of Pride is probably the most notable such example, where you really want to pre-plan who will cover which prisons, and having a random number of targets each cast would cause frustrating unpredictability.
There's nothing wrong with 14-player groups -- 14 players is a great size for a raid. But so is 15. And 19. And 12. And 22. We just want to help make sure that good players aren't being turned away from groups because of the notion that their presence will make things harder for everyone.