Except the boss mechanics aren't going to just dramatically change from week to week. What we know for sure will change is a room's layout. To use their example, there's a boss where you are in a normal room one week so it's standard difficulty. The next week there is a pit of lava in the middle that the boss can be constantly knocked in to give a debuff so he takes more damage making the fight easier.
It's not going to be any different than providing different levels of challenge. Presumably the highest level of raiders will be able to accomplish what they always do, being the best, every week....whereas the lesser skilled/lesser lucked raid teams will not have to wait to farm dungeon points, craft new gear, jump through arbitrary hoops until they can outgear their mistakes.
Instead they might find success in the lava debuff week b/c their mistakes are made up for by the higher RDSP they can put out. So now they get some real progress in a raid setting instead of only having the option to improve outside of a raid. Maybe the handful of drops they get combined with the practice helps them beat it the next week on it's normal setting.
Instead of a static here's the raid, suffer for x months until we nerf it / Here's the raid, beat it in one week then farm until your eyes bleed...it's, here's continual competition that never ends, continual chances at possibly progressing without needing nerfs, etc.