Fire mages were a thing in Naxx and AQ because rolling ignites was kind of broken at the time. The first mage to get the crit was fed by all the other mage's crits... Frost was the only viable spec for MC and BWL though. So that kind of proves that multiple specs were useful. Furthermore, the fact that rolling ignites were so good, and that screenshot you posted shows several mages as Frost spec, clearly something odd was going on, especially on Patchwerk which was a DPS race fight. You can see they are 5 secs from the enrage... They are clearly a casual guild who allow suboptimal specs into their raid.