"Wait. Fire mages weight (sic) haste over crit? When did this happen?"

I can't speak for burning crusade, but it's been better than crit pretty much since 3.01. So that would be just over 2 and a half years.

More haste = more fireballs.

For a while during wrath, arcane was better than fire because of more consistent dps and better scaling. At the end of TOC, beginning of ICC, fire caught up, because crit levels started getting high enough for for more hot streaks, as well as a general buff to fire. At that point, mages, aimed for a balance of crit and haste - basically, the more haste you had, the more crit was valuable, and vice versa. This is why people would say "sim it," because your stat weights were dependent on the stats you had. Stacking haste was basically equivalent to stacking crit, however the advantage of haste with a faster cast means that you can finish your cast before the bad stuff on the floor gets put under your feet.

Once 4.01 hit with dots scaling with haste, it was all over. You stack haste until you hit your "extra dot tick" plateaus, and then go for crit. Once you get the gear enough close to the next plateau, go for the haste to get there.

On top of that you had people hardcasting Pyroblast on some encounters, giving haste an even bigger role to get the cast time of pyroblast down.