Infraction: 10. Do not respond to terrible posts.
Message to User:
Please don't respond to entirely basic questions in a thread meant for advanced theorycrafting. Answering stuff like this in SQA is fine, answering it here is inappropriate -- next time report it.
The reason is simply itemization. Raid gear provides less choice than gear from heroics, and you probably won't be able to get haste pieces in all your slots, especially if you want the set boni (and 4pc t11 is good).