However, even WITHOUT the nerfs to the sons, this change makes it FAR easier for guilds which aren't raiding every night of the week. Why? Because they'll get waaaayyyyyy more time on that boss now. A guild that was getting 3 or 4 attempts in at the end of the raid will be getting 10+ since all the other bosses in there will be down on the first night with time to spare. (domo, baleroc and the stupid bird will all be trivialised beyond belief now)
The easier to kill sons and no more people getting randomly knocked back into lavatraps will just make the fight that little less annoying to learn.
As for why Bliz had to do this, it's simple - they need some sort of gear reset to happen, mostly due to the over-abundance of legendary staffs in top-end guilds, but also because top-end guilds are fully decked out in hard mode gear. This would make balancing the normal mode boss fights a total pain.
They tried nerfing staff droprates, but that just made the problem worse as the top guilds have them already while 10-man guilds are still working on their first. So, they open up hard modes to all, and nerf them so everyone can farm them as if it's normal mode. But they had to do it in the next week or so, to give them time to farm.