Mastery VS Haste with T12 4piece

[Infamoushero]

I know for a good majority of this Expansion Haste seemed to be the stat Hunters reforged into since you received the most DPS from it. I now see that Hunters are reforging away from Haste into Mastery. Is that because the Mastery Proc for MM also procs the T12 4piece? If not what is the reason for Haste being reforged into Mastery. Thank you in advance.

[rinku]

the idea behind that is to reach only specific amounts of haste needed for certain shot rotations.
for example: 1235 haste rating with 3/3 pathing allows you to do CS+4xSS+2xAS in 9.01 seconds cycle on paper, but in reality to reach the same cycle you need slightly more haste to account for latency.