Blizzard started using the PTR (and semi-public beta) for raid testing. This killed any sense of discovery from the raids. The fights are already a known entity for lots of people who smashed their heads on it in the beta. These people made the guides. So the general consensus is - learn the fights before the fights to get every bit of extra advantage that you can.
Blizzard is mostly to blame for this. They are so bad at raid testing that if they kept these raids internal under NDA till patch release, then the first few weeks we would be mostly fixing their bugs. This was a regular occurrence back in the day. Remember the Dark Simulcrum bug on Sinestra or the Saronite Bomb bug on Lich King.
There are other MMO companies which do better job at raid testing. Square Enix keeps all FFXIV boss fights secret till release. E.g. Twintania fight - people had no idea how many phases it had till they actually started progressing through it and going through one phase at a time.
Data-mining sites like MMOC, wowhead are also partly to blame. They just data mine the patch files and figure out abilities and loot before hand kinda forcing Blizzard's hands. So, Blizzard uses top raiding guilds to test these encounters and fine tune them before releasing to the general mass. So WoW has become a different type of MMO - more the race and gear grind kind than the serendipitous kind.
It is a win-win situation for Blizzard really. They dont have to pay to hire quality testers to test out the unfinished beta product. They just open access to the raid bosses to top raiders. The chosen few feel blessed and do Blizzard's testing for them without realizing how much money Blizzard is saving out of not having to hire real testers. It is a beautifully designed business model. Only thing worse is the Star Citizen model - have the consumer pay the company get beta access to actually test out their product.