IV tends to be simplistic in its recommendations, because guides become very complicated if you take account every option.
Haste *is* the best stat for pure single target. The problem is, of course, there's hardly any pure single-target fights in the raid (Shad being the only one, really). Anything where you get incidental cleave will favor Mastery more, and the more targets are involved the better it becomes. Since a lot of the fights - especially the later (and thus more difficult) ones - involve multiple targets, most people will socket for Mastery instead of Haste.
If you really want to know for sure for your given setup, the answer is always: sim yourself. But be careful to have the appropriate settings for your sim